PHP下载代码无法在移动设备上运行

PHP download is not working on mobile devices but it is perfectly working on PC.. This is my code...

session_start();

 $path      = 'editedPic/';
 $image         = $_REQUEST['image'];
 $mainimage  = $path.$image; 
 $username   = $_SESSION['username'];
 $string    = preg_replace('/\s+/', '', $username);

 header('Content-Type: image/jpg');
 header('Content-Disposition: attachment; filename="'.$string.'.jpg"');

 readfile($mainimage);