Convert base64 string to file

The FromBase64String() method converts a base64-encoded string to a byte array. All you need to do is write that byte array back to a file:

$b64      = 'AAAAAA...'
$filename="C:\path\to\file"

$bytes = [Convert]::FromBase64String($b64)
[IO.File]::WriteAllBytes($filename, $bytes)