居然发现文件名编码后长度超过155就会不能正确显示和下载，最后只好找了这样一个折中的方法，截短了
下面是那里的代码
/// <summary>
  /// 下载附件。
  /// </summary>
  /// <param name="fileName">文件名</param>
  /// <param name="path">文件路径</param>
  public static void DownLoadFileAttachment(string fileName , string path)
  {
   if (System.IO.File.Exists(path))
   {
    try
    {
     fileName  = fileName.Trim();

     for (int  i = 0 ; i < System.IO.Path.InvalidPathChars.Length ; i ++)
     {
      fileName = fileName.Trim().Replace(System.IO.Path.InvalidPathChars[i].ToString() , string.Empty);
     }

     fileName  = fileName.Replace(System.IO.Path.PathSeparator.ToString() , string.Empty);
     
     int maxLength = 155;

     int length  = HttpUtility.UrlEncode(fileName).Length;
     while (length > maxLength)
     {
      int index = fileName.LastIndexOf(".");
      if (index > 0)
      {
       fileName = fileName.Substring(0 , index - 1) + fileName.Substring(index);
      }
      else
      {
       fileName = fileName.Substring(0 , fileName.Length - 1);
      }
      length  = HttpUtility.UrlEncode(fileName).Length;
     }

     System.IO.FileInfo file = new System.IO.FileInfo(path);
     HttpContext.Current.Response.Clear();
     HttpContext.Current.Response.AppendHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName));
     HttpContext.Current.Response.AppendHeader("Content-Length", file.Length.ToString());
     HttpContext.Current.Response.ContentType = "application/octet-stream";
     HttpContext.Current.Response.WriteFile(file.FullName);
     HttpContext.Current.Response.End();
    }
    catch
    {
    }
   }
   else
   {
    HttpContext.Current.Response.Clear();
    DisplayNoFileMessage();
    HttpContext.Current.Response.End();
   }
  }