public ActionResult Upload()
    {
      // var pathUrl = "http://" + Request.Url.Authority;
      var file = Request.Files["Filedata"];
      var uploadFileName = file.FileName;
      string filePath = "/File/" + uploadFileName;
      string AbsolutePath = Server.MapPath(filePath);
      file.SaveAs(AbsolutePath);       //将上传的东西保存     
      return Json(new { FileName = uploadFileName, FilePath = filePath });
    }

public ActionResult DownLoad(string FileName)
    {
      string fileName = FileName;//客户端保存的文件名 
      string filePath = Server.MapPath("/File/"+ FileName);//路径    
                                 //以字符流的形式下载文件   
      FileStream fs = new FileStream(filePath, FileMode.Open);
      byte[] bytes = new byte[(int)fs.Length];
      fs.Read(bytes, 0, bytes.Length);
      fs.Close();
      Response.ContentType = "application/octet-stream";
      //通知浏览器下载文件而不是打开  
      Response.AddHeader("Content-Disposition", "attachment; filename=" + HttpUtility.UrlEncode(fileName, System.Text.Encoding.UTF8));
      Response.BinaryWrite(bytes);
      Response.Flush();
      Response.End();
      return Json("");
    }