MVC与DWZ整合中部分问题的解决

[HttpPost]
        [Description("接收商户标题图片")]
        public string UploadTitle(HttpPostedFileBase upImg, string id)
        {
            int MaxSize = 597152;     // 最大上传大小，默认是0.5M
            string newFile = "";
            string ext = "";
            string fn = "";
            string tmp = "";
            try
            {
                //HttpFileCollectionBase fc = Request.Files;
                //HttpPostedFileBase f = Request.Files[0];
                if (upImg.ContentLength > MaxSize || upImg.ContentLength == 0)
                {
                    tmp = "{ \"statusCode\" : \"200\", " +
                               "\"message\" : \"上传失败！文件大小不正确，必须是在0字节以上，0.5M字节以内\"," +
                               "\"navTabId\" : \"\"," +
                               "\"rel\" : \"\"," +
                               "\"callbackType\" : \"closeCurrent\"," +
                               "\"forwardUrl\": \"\"" +
                               "}";
                    return tmp;
                }
                if (!Directory.Exists(Server.MapPath("~/Upload")))
                    Directory.CreateDirectory(Server.MapPath("~/Upload"));
                ext = upImg.FileName.Substring(upImg.FileName.LastIndexOf("."));
                fn = DateTime.Now.ToString("yyyMMddHHmmssfff");
                newFile = Server.MapPath("~/Upload") + "\\" + fn + ext;
                upImg.SaveAs(newFile);
                int dbid = Convert.ToInt32(id);
                var data = db.Seller.Where(p => p.id == dbid).FirstOrDefault();
                if (data != null)
                {
                    data.TitleJPG = "/Upload/" + fn + ext;
                    db.SaveChanges();
                }
                //AJAX不支持上传文件，所以用了隐藏iframe方式，但返回就不能是JSON格式，只能返回字符串格式
                tmp = "{ \"statusCode\" : \"200\", " +
                                "\"message\" : \"修改成功\"," +
                                "\"navTabId\" : \"dlg_modiSeller\"," +
                                "\"rel\" : \"\"," +
                                "\"callbackType\" : \"closeCurrent\"," +
                                "\"forwardUrl\": \"\""+
                                "}";
                return tmp;
                
            }
            catch (Exception ex)
            {
                tmp = "{ \"statusCode\" : \"200\", " +
                               "\"message\" : \"上传图片失败\"," +
                               "\"navTabId\" : \"\"," +
                               "\"rel\" : \"\"," +
                               "\"callbackType\" : \"closeCurrent\"," +
                               "\"forwardUrl\": \"\"" +
                               "}";
                return tmp;
            }

        }

<form action="@Url.Action("UploadTitle", "Seller", new { id = ViewData["id"].ToString() })" method="post" enctype="multipart/form-data" class="pageForm required-validate" onsubmit="return iframeCallback(this,dialogAjaxDone);" >
    <input type="file" name="upImg" id="upImg" style="width: 180px" />
    <button type="submit">文件上传</button>
</form>