asp.net 点击“下载”弹出“下载文件”对话框，网页与要下载的文件不在同一服务器上

#region private void FileDownLoad(string filename)
  /// <summary>
  /// 用户下载文件
  /// </summary>
  /// <param name="filename">下载文件的路径</param>
  private void FileDownLoad(string filename)
  {
  string destFileName = filename;
  destFileName = Server.MapPath("./") + destFileName;
  destFileName = Server.UrlDecode(destFileName);

  if (File.Exists(destFileName))
  {
  FileInfo fi = new FileInfo(filename);
  Response.Clear();
  Response.ClearHeaders();
  Response.Buffer = true;
  Response.Charset = "GB2312";

  //添加头信息，为 "文件下载/另存为 "对话框指定默认文件名 
  Response.AppendHeader("Content-Disposition", "attachment;filename="
  + HttpUtility.UrlEncode(Path.GetFileName(destFileName),
  System.Text.Encoding.UTF8));
  Response.AppendHeader("Content-Length", fi.Length.ToString());
  Response.ContentType = "text/plain";
  Response.Filter.Close();
  Response.WriteFile(destFileName);
  Response.Flush();
  Response.End();
  }
  else
  {
  Response.Write("<script language = javascript>alert('下载出错')</script>");
  }
  }
  #endregion