PSGopher

1.3.1

src/PSGopher.psm1

<#
PSGopher -- a PowerShell client for Gopher and Gopher+ servers.
Copyright (C) 2021-2022 Colin Cogle <colin@colincogle.name>

This program is free software: you can redistribute it and/or modify it under
the terms of the GNU Affero General Public License as published by the Free
Software Foundation, either version 3 of the License, or (at your option) any
later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License along
with this program. If not, see <https://www.gnu.org/licenses/>.
#>

#Requires -Version 7.1
Function Invoke-GopherRequest {
    [CmdletBinding(DefaultParameterSetName='ToScreen')]
    [OutputType([PSCustomObject], ParameterSetName='ToScreen')]
    [OutputType([Void], ParameterSetName='OutFile')]
    [Alias('igr')]
    Param(
        [Parameter(Mandatory, Position=0)]
        [Alias('Url')]
        [ValidateNotNullOrEmpty()]
        [ValidatePattern('^(gophers?|sgopher|gopher\+tls):\/\/')]
        [Uri] $Uri,

        [Alias('UseTLS', 'RequireTLS', 'RequireSSL')]
        [Switch] $UseSSL,

        [Alias('TryTLS', 'OpportunisticTLS', 'OpportunisticSSL')]
        [Switch] $TrySSL,

        [Alias('Abstract','Admin','Attributes','Information')]
        [Switch] $Info,

        [ValidatePattern("[a-z]+\/.+")]
        [AllowNull()]
        [String[]] $Views,

        [ValidateSet('ASCII','UTF7','UTF8','UTF16','Unicode','UTF32')]
        [String] $Encoding = 'UTF8',

        [Parameter(ParameterSetName='OutFile')]
        [ValidateNotNullOrEmpty()]
        [String] $OutFile,

        [Parameter(ValueFromPipeline, ValueFromPipelineByPropertyName)]
        [Alias('Post','PostData','Query','QueryString')]
        [AllowNull()]
        [String] $InputObject
    )

    #region Establish TCP connection.
    # If we have a secure URL scheme, set UseSSL to true.
    $UseSSL = $UseSSL -or ($Uri.Scheme -In @('gophers','sgopher','gopher+tls'))
    
    # Sometimes, the .NET runtime doesn't recognize which port we're supposed
    # to be using -- especially if we use a Gopher-TLS scheme for a secure
    # connection. If so, we need to make a new URL with the port defined.
    If ($Uri.Port -eq -1) {
        $Path, $Query = $Uri.PathAndQuery -Split '\?',2
        $Uri = [Uri]::new("$($Uri.Scheme)://$($Uri.Host):70$Path$($Query ? "?$Query" : '')")
        Write-Debug "New URL = $Uri"
    }

    Write-Verbose "Connecting to $($Uri.Host)$($UseSSL ? ' securely' : '')"
    Try {
        $TcpSocket = [Net.Sockets.TcpClient]::new($Uri.Host, $Uri.Port ?? 70)
        $TcpStream = $TcpSocket.GetStream()
        $TcpStream.ReadTimeout = 2000 #milliseconds
        If ($UseSSL -or $TrySSL) {
            Write-Debug 'Upgrading connection to TLS.'
            $secureStream = [Net.Security.SslStream]::new($TcpStream, $false)
            $secureStream.AuthenticateAsClient($Uri.Host)
            $TcpStream = $secureStream
        }
    }
    Catch {
        $msg = "Could not connect to $($Uri.Host):$($Uri.Port ?? 70)"
        If ($UseSSL -or $TrySSL) {
            $msg += ' with SSL/TLS'
        }

        # If we're using -TrySSL, then we'll retry without encryption. Else,s
        # If we're using -UseSSL, then fail the connection.
        If ($UseSSL) {
            $msg += '. Aborting.'

            # Throw a non-terminating error so that $? is set properly and the
            # pipeline can continue. This will allow chaining operators to work as
            # intended. Should a future version of this module support pipeline
            # input, that will let this cmdlet keep running with other input URIs.
            $er = [Management.Automation.ErrorRecord]::new(
                [Net.WebException]::new($msg),
                'TlsConnectionFailed',
                [Management.Automation.ErrorCategory]::ConnectionError,
                $Uri
            )
            $er.CategoryInfo.Activity = 'NegotiateTlsConnection'
            $PSCmdlet.WriteError($er)
            Return $null
        }
        ElseIf ($TrySSL) {
            Write-Verbose "$msg. Retrying with a non-secured connection."
            $NewParameters = @{
                'Uri' = $Uri
                'Info' = $Info
                'Views' = $Views ?? @()    # not sure why this is needed
                'Encoding' = $Encoding
                'InputObject' = $InputObject
                'TrySSL' = $null
                'UseSSL' = $null
            }

            If ($PSCmdlet.ParameterSetName -eq 'OutFile') {
                $NewParameters.'OutFile' = $OutFile
            }

            Remove-Variable -Name 'SecureStream' -Force
            Return (Invoke-GopherRequest @NewParameters)
            Exit
        }
    }
    #endregion (Establish TCP connection)


    #region Content type negotiation
    $ContentTypeExpected = $null

    # If the user provided one, we'll use that.
    # But it needs to be removed from the URI.
    If ($Uri.PathAndQuery -CMatch "^\/[0123456789+gIT:;<dhis]") {
        $ContentTypeExpected = $Uri.PathAndQuery[1]

        # The code may have removed the leading slash. Put that back.
        # If there was already one, remove it.
        $Path = $Uri.PathAndQuery.Substring(2)
        $Path = "/$Path" -Replace '//','/'

        Write-Debug "Stripped content type: was=$($Uri.PathAndQuery), now=$Path"

        $Uri = [Uri]::new("$($Uri.Scheme)://$($Uri.Host):$($Uri.Port)$Path")
    }

    # Otherwise, let's try and guess -- if we have a file extension.
    ElseIf ($Uri.AbsolutePath -Match '\.') {
        $ContentTypeExpected = (Get-GopherType ($Uri.AbsolutePath -Split '\.')[-1] -Verbose:$VerbosePreference -Debug:$DebugPreference)
    }

    # If we still can't figure it out after all this, assume it's a Gopher menu.
    $ContentTypeExpected ??= 1

    # Determine if we're reading a binary file or text.
    $BINARY_TRANSFER = (-Not $Info) -and ($ContentTypeExpected -In @(4,5 ,9,'g','I',':',';','<','d','s') )
    #endregion (Content type negotiation)

    #region Parse input parameters
    If ($null -eq $InputObject -or $InputObject.Length -eq 0) {
        Write-Debug 'No additional query string detected.'
    }
    Else {
        Write-Debug "Found additional query string=$InputObject"

        $Encoder = [Web.HttpUtility]::ParseQueryString('')
        $Encoder.Add($null, $InputObject)
        $EncodedInput = $Encoder.ToString() -Replace '\+','%20'    # Gopher requires URL (percent) encoding for spaces.
        
        Write-Debug "Encoded additional query string=$EncodedInput"

        # If there was already a query string specified in the URL, we will send
        # both of them, with the URL taking precedence.
        If ($Uri.Query) {
            Write-Debug "Found existing query string=$($Uri.Query)"
        }
        $Uri = [Uri]::new($Uri.ToString() + ($Uri.Query ? '&' : '?') + $EncodedInput)            
    }
    #endregion

    #region Send request
    $ToSend = $Uri.PathAndQuery
    If ($Info) {
        If ($ContentTypeExpected -eq 1) {
            $ToSend += "`t$"
        }
        Else {
            $ToSend += "`t!"
        }
    }
    If ($Views) {
        $ToSend += "`t+$Views"
    }
    $ToSend += "`r`n"
    Write-Debug "Sending $($ToSend.Length) bytes to server: $($ToSend -Replace "`r",'\r' -Replace "`n",'\n' -Replace "`t",'\t')"
    $writer = [IO.StreamWriter]::new($TcpStream)
    $writer.WriteLine($ToSend)
    $writer.Flush()
    #endregion (Send request)

    #region Receive data
    # Set text encoding for reading and writing textual output.
    If (-Not $BINARY_TRANSFER) {
        Switch ($Encoding) {
            'ASCII'   {$Encoder = [Text.AsciiEncoding]::new()}
            'UTF7'    {$Encoder = [Text.UTF7Encoding]::new()}
            'UTF8'    {$Encoder = [Text.UTF8Encoding]::new()}
            'UTF16'   {$Encoder = [Text.UnicodeEncoding]::new()}
            'Unicode' {$Encoder = [Text.UnicodeEncoding]::new()}
            'UTF32'   {$Encoder = [Text.UTF32Encoding]::new()}
            default   {Throw [NotImplementedException]::new('An unknown Encoder was specified.')}
        }
    }

    # Read the full response.
    $response = ($BINARY_TRANSFER ? [IO.MemoryStream]::new() : '')
    $BufferSize = 102400     # 100 KB, more than enough for text, but a sizable
                            # buffer to make binary transfers fast.
    $buffer = New-Object Byte[] $BufferSize

    If (-Not $BINARY_TRANSFER)
    {
        If ($Info) {
            Write-Debug "Beginning to read (attributes)."
        } Else {
            Write-Debug "Beginning to read (textual type $ContentTypeExpected)."
        }
        
        While (0 -ne ($bytesRead = $TcpStream.Read($buffer, 0, $BufferSize))) {
            Write-Debug "`tReading ≤$BufferSize bytes from the server."
            $response += $Encoder.GetString($buffer, 0, $bytesRead)
        }
        Write-Verbose "Received $($Encoder.GetByteCount($response)) bytes from server."
    }
    Else # it is a binary transfer #
    {
        Write-Debug "Beginning to read (binary type $ContentTypeExpected)."
        While (0 -ne ($bytesRead = $TcpStream.Read($buffer, 0, $BufferSize))) {
            Write-Debug "`tRead ≤$BufferSize bytes from the server."
            $response.Write($buffer, 0, $bytesRead)
        }
        $response.Flush()
        Write-Verbose "Received $($response.Length) bytes from server."
    }
    #endregion (Receive data)

    # Close connections.
    Write-Debug 'Closing connections.'
    $writer.Close()
    $TcpSocket.Close()

    #region Parse response
    $Content = ''
    $Links = @()

    # Check for errors. All errors begin with '3'.
    If ( `
        ($BINARY_TRANSFER -and $response.ToArray()[0] -eq 51) -or `
        (-Not $BINARY_TRANSFER -and $response[0] -eq '3' -and $response -CLike '*error.host*') `
    ) {
        If ($BINARY_TRANSFER) {
            $response = [Text.Encoding]::ASCII.GetString($response.ToArray())
        }
        Write-Error -Message ($response.Substring(1, $response.IndexOf("`t"))) -TargetObject $Uri -ErrorId 3 -Category 'ResourceUnavailable'
        Return $null
    }
    # If this is not a Gopher menu, then simply return the raw output.
    ElseIf ($BINARY_TRANSFER) {
        $Content = $response.ToArray()
    }
    # If this is anything non-binary and not a menu, simply return it.
    ElseIf ($ContentTypeExpected -ne 1) {
        $Content = $response
    }
    Else {
        $response -Split "(`r`n)" | ForEach-Object {
            Write-Debug "OUTPUT: $($_ -Replace "`r",'' -Replace "`n",'')"

            # Build Content variable
            If ($_.Length -gt 0) {
                $Content += ($_.Substring(1) -Split "`t")[0]
            }
            Else {
                $Content += "`r`n"
            }

            # Look for links or errors. However, we can skip this if we're using
            # the -OutFile or -Info parameters, because no link objects are returned.
            If (-Not $OutFile -and -Not $Info  -and $_ -Match "`t") {
                $line = $_
                Switch -RegEx ($_[0]) {
                    'i' {
                        Break
                    }

                    default {
                        $result = Convert-GopherLink $line -Server $Uri.Host -Port $Uri.Port -Verbose:$VerbosePreference -Debug:$DebugPreference
                        $Links += $result
                    }
                }
            }
        }
    }
    #endregion (Parse response)

    #region Generate output
    # If we are saving the output to a file, then we do not send anything to the
    # output buffer. We save the Content to a file instead.
    If ($OutFile) {
        # Don't write output if an error occurred.
        If ($response[0] -eq '3') {
            Write-Error $Content
            Return $null
        } Else {
            If (-Not $BINARY_TRANSFER)
            {
                Write-Verbose "Writing $($Encoder.GetByteCount($response)) bytes to $OutFile"
                Set-Content -Path $OutFile -Value $Content -Encoding $Encoding 
            }
            Else {
                Write-Verbose "Writing $($response.Length) bytes to $OutFile"
                Set-Content -Path $OutFile -Value $Content -AsByteStream 
            }
        }
        Return
    }
    # TODO: figure out how to parse Gophermaps in Gopher+ mode.
    # For now, let's skip all this and return it as plain text.
    ElseIf ($Info -and $ContentTypeExpected -ne 1) {
        $Result = [PSCustomObject]@{}

        # For each line of Gopher+ output, we're going to see if it begins with
        # a plus sign. If so, we have an attribute name. Then, we're going to
        # go through each line of output and save that. Once we find another
        # attribute name, add the two items to $Result.
        $AttributeName = ''
        $AttributeValue = ''
        $response -Split "(\+[A-Z]+):" | ForEach-Object {
            If ($_.Length -gt 0) {
                Write-Debug "Gopher+ output line: $_"
                # If we've found an attribute, then add the current name/value
                # into $Result (if there is one).
                If ($_[0] -eq '+') {
                    If ($AttributeValue) {
                        If ($AttributeName -In @('ADMIN', 'VIEWS')) {
                            $splits = $AttributeValue.Split("`r`n", [StringSplitOptions]::RemoveEmptyEntries).Trim()
                            $Result | Add-Member -NotePropertyName $AttributeName -NotePropertyValue $splits
                            # ($AttributeValue -Split "\s*\r\n\s*")
                        }
                        Else {
                            $Result | Add-Member -NotePropertyName $AttributeName -NotePropertyValue $AttributeValue.Trim()
                        }
                    }

                    # Now, get ready for the next attribute.
                    $AttributeName  = $_.Substring(1).Trim()
                    $AttributeValue = ''
                }
                # This is not an attribute name, so add it to our
                # currently-saved value.
                Else {
                    $AttributeValue += $_
                }
            }
        }

        # What's left in $AttributeValue must be an attribute.
        # This is a repeat of the above few lines of code. If anyone can
        # refactor this into something better, please do!
        If ($AttributeName -In @('ADMIN', 'VIEWS')) {
            $Result | Add-Member -NotePropertyName $AttributeName -NotePropertyValue $AttributeValue.Split("\s*(\r\n)+\s*", [StringSplitOptions]::RemoveEmptyEntries)
        }
        Else {
            $Result | Add-Member -NotePropertyName $AttributeName -NotePropertyValue $AttributeValue.Trim()
        }
        Return $Result
    }
    Else {
        # Let's tell the user how we fetched this resource/attributes.
        # This will be more useful when I implement opportunistic TLS.
        $Protocol = 'Gopher'
        If ($Info -or $Views) {
            $Protocol = 'Gopher+'
        }
        If ((Test-Path 'variable:\SecureStream') -and ($null -ne $SecureStream)) {
            $Protocol = "Secure$Protocol"
        }

        Return [PSCustomObject]@{
            'Protocol' = $Protocol
            'ContentType' = $ContentTypeExpected ?? 1
            'Content' = $Content
            'Encoding' = ($BINARY_TRANSFER ? $Content.GetType() : $Encoder.GetType())
            'Images'  = $Links | Where-Object Type -In @('g','I')
            'Links' = $Links
            'RawContent'  = ($BINARY_TRANSFER ? $response.ToArray() : $response)
            'RawContentLength' = $response.Length
        }
    }
    #endregion (Generate output)
}

Function Convert-GopherLink {
    [CmdletBinding()]
    [OutputType([PSCustomObject])]
    Param(
        [Parameter(Mandatory, Position=0)]
        [ValidateNotNullOrEmpty()]
        [ValidatePattern('^(?:.)(?:[^\t]*)\t')]
        [String] $InputObject,

        [String] $Server,

        [UInt16] $Port
    )

    Write-Debug '*** Found a Gopher link.'
    $fields = $InputObject -Split "`t"
    $Uri    = $null

    # Are we dealing with a /URL: link? If so, we can easily create
    # the href [Uri]. Otherwise, we'll need to build it ourselves.
    If ($fields[1] -CLike 'URL:*' -or $fields[1] -CLike '/URL:*') {
        $Uri = [Uri]::new($fields[1] -Replace [RegEx]"\/?URL:",'')
    }
    Else {
        $Server = ${fields}?[2] ?? $Server
        $Port   = ${fields}?[3] ?? $Port

        # Pick the appropriate URL schema for the link type.
        # For the first two (CCSO and Telnet), there should be nothing after the
        # optional port, but let's include it anyway.
        Switch -RegEx ($fields[0][0]) {
            '2'     {$Port ??= 105; $Uri = [Uri]::new("cso://${Server}:$Port/$($fields[1])")}
            '[8T]'  {$Port ??= 23;  $Uri = [Uri]::new("telnet://${Server}:$Port/$($fields[1])")}
            default {$Port ??= 70;  $Uri = [Uri]::new("gopher://${Server}:$Port/$($fields[1])")}
        }
    }

    Write-Debug "*** Type=$($fields[0][0]): $Uri"
    Write-Verbose "LINK: Type=$($fields[0][0]): $Uri"
    Return [PSCustomObject]@{
        'href' = $uri
        'Type' = $fields[0][0]
        'Description' = $fields[0].Substring(1)
        'Resource' = $Uri.AbsolutePath
        'Server' = $Uri.Host
        'Port' = $Uri.Port
        'UrlLink' = ($InputObject -Match '\t\/?URL:')
    }
}

# This helper function guessed at types when the user forgets to enter one.
# This ensures that data will be returned in either text or binary format.
# Feel free to add extensions and types as you see fit.
Function Get-GopherType {
    [CmdletBinding()]
    [OutputType([Char])]
    Param(
        [ValidateNotNullOrEmpty()]
        [String] $Extension
    )

    # This list will be searched case-insensitively.
    $Extensions = @{
        'ace' = 9                # ACE archive
        'ai' = 'I'                # Adobe Illustrator image
        'aif[cf]?' = '<'        # AIFF sound
        'applescript|scpt' = 0    # AppleScript code
        'arj' = 9                # ARJ archive
        'art' = 'I'                # AOL ART image
        'asc' = 0                # GPG data (text)
        'asf' = ';'                # ASF sound
        'asm|s' = 0                # Assembly code
        'ass|ssa|srt' = 0        # Subtitles
        'au' = '<'                # Sound
        'av1' = ';'                # AV1 movie
        'avi' = ';'                # AVI movie
        'avif' = 'I'            # AVIF image
        'bat|cmd' = 0            # Batch file
        'bin' = 9                # Generic binary
        'bmp|dib|pcx' = ':'        # Bitmap image
        'br' = 9                # Brotli-compressed data
        'bz2' = 9                # BZIP2 archive
        'c|h' = 0                # C source code
        'cab' = 9                # Windows cabinet
        'cer' = 0                # Certificate (probably text)
        'cgm' = 'I'                # CGM image
        'coffee' = 0            # CoffeeScript
        'conf|cfg?|ini' = 0        # Config file
        'cpio' = 9                # CPIO archive
        '[ch](?:pp|xx)' = 0        # C++ code
        'crl' = 9                # Certificate revocation list
        'crt' = 9                # Certificate (probably binary)
        '[ch]s' = 0                # C# code
        'css' = 0                # CSS stylesheet
        'csv' = 0                # CSV data
        'cur|ani' = 5            # Windows cursor
        'deb|rpm|apk' = 9        # Linux packages
        'der' = 0                # Certificate (as text)
        'diff' = 0                # diff
        'dll' = 5                # DOS/Windows library
        'dmg|sparseimage' = 9    # macOS disk image
        'dng' = 'I'                # Digital negative
        'dns' = 0                # DNS zone
        'do[ct][mx]?' = 'd'        # Microsoft Word document
        'dsk' = 9                # Disk image
        'dvi' = 'd'                # DVI document
        'dvr-ms' = ';'            # Windows Media Center movie
        'dwg' = 'I'                # AutoCAD image
        'ebuild' = 0            # Gentoo ebuild
        'emf|wmf' = 'I'            # Windows metafile image
        'eml|msg' = 0            # Email message
        'eps' = 'I'                # Vector image
        'epub|mobi' = 9            # Book
        'exe|com|pif' = 5        # DOS/Windows app
        'f?odg|otg' = 'I'        # OpenDocument drawing
        'f?odp|otp' = 'd'        # OpenDocument presentation
        'f?ods|ots' = 'd'        # OpenDocument spreadsheet
        'f?odt|ott' = 'd'        # OpenDocument document
        'fon' = 5                # DOS/Windows font
        'flac' = '<'            # FLAC audio
        'flv' = ';'                # Flash video
        'gif' = 'g'                # GIF image
        'gifv' = ';'            # GIFV video
        'gmi' = 0                # Gemtext
        'gnumeric' = 'd'        # Gnumeric spreadsheet
        'go' = 0                # Go source code
        'gpg' = 9                # GPG data (binary)
        'gz' = 9                # Compressed data
        'hei[cf]' = 'I'            # HEIC image
        'hqx' = 4                # BinHex archive
        'html?' = 'h'            # HTML document
        'icns' = 'I'            # macOS icon
        'ico' = 'I'                # Windows icon
        'img' = 9                # Disk image
        'inf' = 0                # Windows INF file
        'ini' = 0                # Configuration file
        'ipsw' = 9                # iOS/iPod software update
        'iso' = 9                # CD image
        'jar' = 9                # Java app
        'java' = 0                # Java source code
        'jp2' = 'I'                # JPEG 2000 image
        'jpe?g' = 'I'            # JPEG image
        'js' = 0                # JavaScript code
        'json' = 0                # JSON data
        'jsonld' = 0            # JSON-LD data
        'jxl' = 'I'                # JPEG XL image
        'lnk' = 5                # Windows shortcut
        'log' = 0                # Log
        'lua' = 0                # Lua source code
        'lz' = 9                # Compressed data
        'lzh' = 9                # Compressed data
        'lzma' = 9                # Compressed data
        'lzo' = 9                # Compressed data
        'm3u8?' = '<'            # Playlist
        'm4' = 0                # M4 source code
        'm4[abpr]' = '<'        # MPEG-4 audio formats (mostly iTunes)
        'm4v|mp4' = ';'            # MPEG-4 container (usually video)
        'md|markdown' = 0        # Markdown text
        'midi?' = '<'            # MIDI music
        'mkv' = ';'                # Matroska video
        'mov' = ';'                # QuickTime movie
        'mp3' = '<'                # MP3 audio
        'mpe?g' = ';'            # MPEG movie
        'mpp' = 'd'                # Microsoft Project document
        'msp' = 'I'                # Microsoft Paintbrush image
        'numbers' = 'd'            # Numbers spreadsheet
        'o' = 9                    # Object file
        'ocx' = 5                # ActiveX control
        'odb' = 'd'                # OpenDoucment database
        'odf' = 'd'                # OpenDocument formula
        'og[gm]' = '<'            # Ogg audio
        'ogv' = ';'                # Ogg video
        'ovl' = 5                # DOS overlay
        'o?xps' = 'd'            # (Open)XPS document
        'pages' = 'd'            # Pages document
        'par2?' = 9                # PAR archive
        'pas' = 0                # Pascal code
        'pbm' = ':'                # PBM image
        'pi?ct' = 'I'            # Apple PICT image
        'pdf' = 'd'                # PDF document
        'pdn' = 'I'                # Paint.NET image
        'pem' = 0                # PEM-encoded data
        'pfx|p12|p7[bc]' = 9    # Certificates
        'php[345]?' = 0            # PHP code
        'pl' = 0                # Perl code
        'png' = 'I'                # PNG image
        'pptx?|pps|pot' = 'd'    # PowerPoint presentation
        'ps' = 'd'                # PostScript document
        'psd' = 'I'                # Photoshop image
        'psp' = 'I'                # Paint Shop Pro image
        'ps[cdm]?1|ps1xml' = 0    # PowerShell code
        'pub' = 'd'                # Publisher document
        'py' = 0                # Python code
        'py[co]' = 9            # Python bytecode
        'r' = 0                    # R code
        'rar' = 9                # WinRAR archive
        'rb' = 0                # Ruby source code
        'rdp' = 0                # Microsoft Remote Desktop connection
        'rss|atom' = 0            # News feed
        'rtfd?' = 'd'            # Rich Text Format document
        'scr' = 5                # Windows screen saver
        'scss' = 0                # Sass code
        'sh|bash|command' = 0    # Shell script
        'sht(?:ml)?' = 'h'        # HTML with includes
        'sitx?' = 9                # Stuffit archive
        'snd' = '<'                # Sound
        'sql' = 0                # SQL code
        'svgz?' = 'I'            # SVG image
        'sys|drv' = 5            # DOS/Windows driver
        'tab|tsv' = 'd'            # Tab-encoded values
        'tar' = 9                # TAR archive
        'targa|tga' = 'I'        # Targa image
        'tcl' = 0                # TCL code
        'tex' = 0                # Tex code
        'tiff?' = 'I'            # TIFF image
        'tt[cf]|otf|woff2?' = 9    # Font
        'txt' = 0                # Text
        'uue' = 6                # UUEncoded data
        'vbs' = 0                # VBScript
        'vhdx?' = 9                # Windows disk image
        'vsdx?' = 'I'            # Visio drawing
        'wav' = '<'                # Wave audio
        'webm' = ';'            # WebM video
        'webp|wp2' = 'I'        # WebP (2) image
        'wim|esd' = 9            # Windows Image (archive)
        'wma|asf' = ';'            # Windows Media audio
        'wmf|emf' = 'I'            # Windows Metafile image
        'wmv' = ';'                # Windows Media video
        'wpt?' = 'd'            # WordPerfect doucment
        'wri' = 'd'                # Windows Write document
        'xcf' = 'I'                # GIMP image
        'xht(?:ml)?' = 'h'        # XHTML code
        'xl(s[bmx]?|w)' = 'd'    # Excel document
        'xltm?'    = 'd'            # Excel template
        'xml|rdf' = 0            # XML code
        'xpm' = 'I'                # XPM image
        'xsd' = 0                # XSD code
        'xslt?' = 0                # XML stylesheet
        'xz' = 9                # Compressed data
        'ya?ml' = 0                # YAML code
        'Z' = 9                    # Compressed data
        'zip' = 9                # Zip archive
        'zoo' = 9                # ZOO archive
        '123' = 'd'                # Lotus 1-2-3 document
        '7z' = 9                # 7-zip archive
    }

    $Result = $null
    ForEach ($regex in $Extensions.GetEnumerator()) {
        Write-Debug "Testing extension $Extension against $($regex.Name)."
        If ($Extension -Match $regex.Name) {
            $Result = $regex.Value
            Break
        }
    }
    Write-Verbose "Guessing that the extension $Extension is of type $($Result ?? 'unknown')."
    Return $Result
}