https://twitter.com/aetos382/status/1004659626460135424

Get-LonguestExtension.ps1

$keys =
    Get-ChildItem -LiteralPath 'Registry::HKEY_CLASSES_ROOT' |
    Add-Member -MemberType ScriptProperty -Name 'LeafName' -Value  { Split-Path -Leaf $this.PSPath } -Force -PassThru |
    ? { $_.LeafName.StartsWith('.') } |
    % {
        $progIds = @($_.GetValue(''))
        
        $key = Get-Item -LiteralPath (Join-Path -Path $_.PSPath -ChildPath 'OpenWithProgIds') -ErrorAction Ignore
        if ($key) {
            $progIds += @($key.GetValueNames())
        }

        foreach ($progId in $progIds) {
            $progIdKey = Get-Item -LiteralPath "Registry::HKEY_CLASSES_ROOT\${progId}" -ErrorAction Ignore
            if ($progIdKey) {
                $description = $progIdKey.GetValue('')

                if ($description) {
                    $_ | Add-Member -MemberType NoteProperty -Name 'Description' -Value $description -Force
                    break
                }
            }
        }

        $_
    }

$extensions = $keys |
    Sort-Object -Property @{ Expression = { $_.LeafName.Length }; Descending = $true } |
    Select-Object -First 100 -Property 'LeafName', 'Description'

$extensions