// determine which processor to use based on the extension
        $processor = "";

        $allowed_sox = array('mp3','wav','ogg', 'aif', 'aiff', 'flac');
        $allowed_avconv = array('m4a','aac','wma');

        if (in_array($ext, $allowed_sox)) {
            $processor = "sox";
        }

        if (in_array($ext, $allowed_avconv)) {
            $processor = "avconv";
        }

        // Check we have a valid processor
        if ($processor == "") {
            json_response(422, 'No processor support!');
            exit;
        }