Converts a Base64 string into something usable

ConvertFrom-Base64String.ps1

using namespace System.Management.Automation
using namespace System.Text

class EncodingTransformAttribute : ArgumentTransformationAttribute {

    [object] Transform([EngineIntrinsics]$engineIntrinsics, [object] $inputData) {
        
        switch ($inputdata) {
            { $_ -is  [System.Text.Encoding]} {

                return $_
            }

            { $_ -is [String]} {
                if ( $_ -in ([Encoding] | Get-Member -Static -MemberType Property).Name ) {
                  return [System.Text.Encoding]::$_
                }
                elseif ($_ -in [System.Text.Encoding]::GetEncodings().Name) {

                    return [System.Text.Encoding]::GetEncoding($_)
                }
            }
        }
        
        throw [ArgumentTransformationMetadataException]::new()
    }
}

function ConvertFrom-Base64String {
    <#
        .SYNOPSIS
        Converts a Base64 String into human readable text in your default encoding

        .PARAMETER Base64String

        .EXAMPLE
        ConvertFrom-Base64String "kRlc2NyaXB0aW9uIjogICJKdXN0IHRlc3"

        .EXAMPLE
        "kRlc2NyaXB0aW9uIjogICJKdXN0IHRlc3" | ConvertFrom-Base64String
        
    #>
    [cmdletBinding()]
    param(
        [Parameter(Mandatory,ValueFromPipeline,ValueFromPipelineByPropertyName)]
        [String]
        $Base64String,

        [Parameter()]
        [ArgumentCompleter(
            {
                param($Command, $Parameter, $Encoding, $CommandAst, $FakeBoundParams)
                $ValidEncodings = ([Encoding] | Get-Member -Static -MemberType Property).Name + [System.Text.Encoding]::GetEncodings().Name | Sort-Object
                
                if(!$Encoding){
                    return $ValidEncodings
                }
                else {
                    return $ValidEncodings.Where{ $_.StartsWith($Encoding) }
                }
            }
        )]
        [EncodingTransformAttribute()]
        [Encoding]
        $Encoding
    )

    begin {}

    process {

        $Encoding.GetString([System.Convert]::FromBase64String($Base64String))
    }

    end {}

}