I’m pretty sure you don’t add the mime type as a JPEG on file downloads:
header('Content-Type: image/png');
These headers have never failed me:
$quoted = sprintf('"%s"', addcslashes(basename($file), '"\\'));
$size = filesize($file);
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename=" . $quoted);
header("Content-Transfer-Encoding: binary');
header('Connection: Keep-Alive');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . $size);