Some example code to rename discord cached images

Rename-DiscordCacheImagesToExt

# Function to figure out what the file type is
function Get-ImageExt {
    [CmdletBinding()]
    # [OutputType([System.Boolean])]
    param(
        [Parameter(Mandatory = $true, Position = 0, ValueFromPipeline = $true)]
        [ValidateNotNullOrEmpty()]
        [Alias('PSPath')]
        [string] $Path
    )
    PROCESS {
        $knownHeaders = @{
            jpg = @( "FF", "D8" );
            bmp = @( "42", "4D" );
            gif = @( "47", "49", "46" );
            tif = @( "49", "49", "2A" );
            png = @( "89", "50", "4E", "47", "0D", "0A", "1A", "0A" );
            pdf = @( "25", "50", "44", "46" );
        }
        # coerce relative paths from the pipeline into full paths
        if ($_ -ne $null) {
            $Path = $_.FullName
        }
        # read in the first 8 bits
        $bytes = Get-Content -LiteralPath $Path -Encoding Byte -ReadCount 1 -TotalCount 8 -ErrorAction Ignore
        $retval = $false
        foreach ($key in $knownHeaders.Keys) {
            # make the file header data the same length and format as the known header
            $fileHeader = $bytes |
                Select-Object -First $knownHeaders[$key].Length |
                ForEach-Object { $_.ToString("X2") }
            if ($fileHeader.Length -eq 0) {
                continue
            }
            # compare the two headers

            $diff = Compare-Object -ReferenceObject $knownHeaders[$key] -DifferenceObject $fileHeader
            if (($diff | Measure-Object).Count -eq 0) {
                $diff
                $retval = $true
            }
        }
        switch ($knownHeaders[$key].Values) {
            $knownHeaders[0].Values { return "jpg" }
            $knownHeaders[1].Values { return "bmp" }
            $knownHeaders[2].Values { return "gif" }
            $knownHeaders[3].Values { return "tif" }
            $knownHeaders[4].Values { return "png" }
            $knownHeaders[5].Values { return "pdf" }
            Default {}
        }
        return $retval
    }
}

# Rename Discord images with extensions
Get-Item "$($env:APPDATA)\discord\Cache\f_*" | ForEach-Object {
    $ext = Get-ImageExt -Path $_
    Rename-Item -Path $_ -NewName "$_.$ext"
}

Rename-DiscordCacheImagesToOriginal

# Remove Discord cache extentions
Get-Item "$($env:APPDATA)\discord\Cache\f_*" | ForEach-Object {
    Rename-Item -Path $_ -NewName $_.BaseName
}