MVC上传文件

<form class="cmxform form-horizontal tasi-form" id="submitForm" method="POST" action="Add" novalidate="novalidate" enctype="multipart/form-data">
                        @{
                            if (!ViewData.ModelState.IsValid)
                            {
                                <div class="alert alert-danger alert-dismissable">
                                    <button type="button" class="close" data-dismiss="alert" aria-hidden="true">×</button>
                                    @string.Format("操作失败：{0}", ViewData.ModelState["Error"].Errors[0].ErrorMessage)
                                </div>
                            }
                        }

                        <div class="form-group ">
                            @Html.LabelFor(item => item.Name, new { @class = "control-label col-lg-2" })
                            <div class="col-lg-10">
                                @Html.TextBoxFor(item => item.Name, new { @class = "form-control"})
                            </div>
                        </div>
                        <div class="form-group ">
                            @Html.LabelFor(item => item.LinkUrl, new { @class = "control-label col-lg-2" })
                            <div class="col-lg-10">
                                @Html.TextBoxFor(item => item.LinkUrl, new { @class = "form-control" })
                            </div>
                        </div>

                        <div class="form-group ">
                            @Html.LabelFor(item => item.Img, new { @class = "control-label col-lg-2" })
                            <div class="col-lg-10">
                                <input type="file" name="file" class="form-control" />
                            </div>
                        </div>

                        <div class="form-group">
                            <div class="col-lg-offset-2 col-lg-10">
                                <button class="btn btn-success" type="submit">保存</button>
                                <button class="btn btn-default" type="button" onclick="window.history.go(-1);">返回</button>
                            </div>
                        </div>
                    </form>

[HttpPost]
        public ActionResult Add(Service.Dto.BannerInfoDto dto, HttpPostedFileBase file)
        {
            string fileName = "";

            if (file == null)
            {
                ModelState.AddModelError("Error", "请选择图片 .jpg/.png/.gif");
                return View(dto);
            }

            if (file != null)
            {
                string fileExt = Path.GetExtension(file.FileName).ToLower();
                if (!".jpg/.png/.gif".Contains(fileExt))
                {
                    ModelState.AddModelError("Error", "图片格式不正确 .jpg/.png/.gif");
                    return View(dto);
                }

                fileName = "~/UploadFile/BannerInfo/" + DateTime.Now.ToString("yyyyMMddHHmmss") + fileExt;

                string serPath = Server.MapPath(fileName);

                try
                {
                    file.SaveAs(serPath);
                }
                catch (Exception ex)
                {
                    ModelState.AddModelError("Error", "上传异常：" + ex.Message);
                    return View(dto);
                }
            }

            Domain.RL_BannerInfo model = new Domain.RL_BannerInfo();
            model.LinkUrl = dto.LinkUrl;
            model.Name = dto.Name;
            model.Img = fileName;

            bool result = BannerInfoManage.Save(model); ;
            if (result)
            {
                return RedirectToAction("Manage");
            }
            else
            {
                ModelState.AddModelError("Error", "保存失败");
                return View(dto);
            }
        }