decode base64 to file without MIME

Question

I receive file from android as base64 without MIME data:image/png;base64, how to get file extension my code

    $output_file = '';//location file in server and extension
    $base64_string = '';//code base64
    $ifp = fopen($output_file, "wb"); 
    fwrite($ifp, base64_decode($base64_string)); 
    fclose($ifp);