function fix_gall($content){
preg_match_all('#(\[gallery.+?\])#is',$content,$listgall);
foreach($listgall[1] as $gall){
 if (stripos($gall,'link=') === false) $content = str_replace($gall, substr($gall, 0, -1).' link="file"]', $content);
}
return $content ;
}
add_filter('content_save_pre','fix_gall');