I\'m using typical PHP code to download documents:
  header(\'Content-Type: \' . $mimeTypes[$fileext]); 
  header(\'Content-Disposition: attachment; filename         
        Add this to your headers:
header("Content-type: application/force-download");
                                                                        The following php worked for me
header("Cache-Control: private");
header("Content-Description: File Transfer");
header("Content-Disposition: attachment; filename=".$file."");
header("Content-Transfer-Encoding: binary");
header("Content-Type: binary/octet-stream"); 
readfile ($link);
P.S. - I used the File extension in the $file variable only, so no need to mention mime type...
Hope this helps :)