php下载图片到本地，图片打不开,该怎么处理

function dl_file($file){
     //这是下载文件的函数,$file是文件路径.
    //First, see if the file exists
    if (!is_file($file)) { die("404 File not found!"); }

    //Gather relevent info about file
    $len = filesize($file);
    $filename = basename($file);
    $file_extension = strtolower(substr(strrchr($filename,"."),1));

    //This will set the Content-Type to the appropriate setting for the file
    switch( $file_extension ) {
          case "pdf": $ctype="application/pdf"; break;
      case "exe": $ctype="application/octet-stream"; break;
      case "zip": $ctype="application/zip"; break;
      case "doc": $ctype="application/msword"; break;
      case "xls": $ctype="application/vnd.ms-excel"; break;
      case "ppt": $ctype="application/vnd.ms-powerpoint"; break;
      case "gif": $ctype="image/gif"; break;
      case "png": $ctype="image/png"; break;
      case "jpeg":
      case "jpg": $ctype="image/jpg"; break;
      case "mp3": $ctype="audio/mpeg"; break;
      case "wav": $ctype="audio/x-wav"; break;
      case "mpeg":
      case "mpg":
      case "mpe": $ctype="video/mpeg"; break;
      case "mov": $ctype="video/quicktime"; break;
      case "avi": $ctype="video/x-msvideo"; break;

      //The following are for extensions that shouldn't be downloaded (sensitive stuff, like php files)
      case "php":
      case "htm":
      case "html":
      case "txt": die("Cannot be used for ". $file_extension ." files!"); break;

      default: $ctype="application/force-download";
    }

    //Begin writing headers
    header("Pragma: public");
    header("Expires: 0");
    header("Cache-Control: must-rev alidate, post-check=0, pre-check=0");
    header("Cache-Control: public"); 
    header("Content-Description: File Transfer");
    
    //Use the switch-generated Content-Type
    header("Content-Type: $ctype");

    //Force the download
    $header="Content-Disposition: attachment; filename=".$filename.";";
    header($header);
    header("Content-Transfer-Encoding: binary");
    header("Content-Length: ".$len);
    @readfile($file);
    exit;
}