PHP offers us the perfect solution to force the download of a file and propose to the user a filename:

$file ='monkey.gif';
if (file_exists($file)) {
  header('Content-Description: File Transfer');
  header('Content-Type: application/octet-stream');
  header('Content-Disposition: attachment; filename='.basename($file));
  header'Content-Transfer-Encoding: binary');
  header('Expires: 0');
  header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
  header('Pragma: public');
  header('Content-Length: '. filesize($file));

  ob_clean();
  flush();
  readfile($file);
  exit;
}

via PHP: readfile – Manual.