对于WAV和ZIP文件，Android上的下载大小不正确

I've created a simple download script that works flawless on desktop browsers. However when trying to download ZIP or WAV files on Android it seems to show and download only 36% of the file.. Is this a known error or am I doing something completely wrong here?

My function triggered after checks:

function forceDownload($file_to_download,$header){

    header($_SERVER['SERVER_PROTOCOL'].' 200 OK');
    header("Content-Type: ".$header);
    header("Content-Transfer-Encoding: Binary");
    header('Cache-Control: must-revalidate');
    header("Pragma: public");
    header("Content-Length: ".filesize($file_to_download));
    header("Content-Disposition: attachment;
    filename=\"".basename($file_to_download)."\"");
    ob_end_flush();
    @readfile($file_to_download);
    exit();

}