asp.net MVC上传文件在action的参数中获取null

Question

我正在尝试上传文件。我按照这些例子：

http://haacked.com/archive/2010/07/16/uploading-files-with-aspnetmvc.aspx/ http://www.mikesdotnetting.com/article/259/asp-net-mvc-5-with-ef-6-working-with-files

当我在编辑操作中检查文件时，它为空。我哪里做错了？这是我的代码：

控制器：

[HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult Edit(int? id, FormCollection form, string addFileButton, HttpPostedFileBase file)
        {
            var cardKey = db.CardKeys.Include(c => c.Request).Single(c => c.CardKeyID == id);

            if (addFileButton != null)
            {
                if (file != null && file.ContentLength > 0)
                {
                    var fileName = Path.GetFileName(file.FileName);
                    var path = Path.Combine(Server.MapPath("~/App_Data/uploads"), fileName);
                    file.SaveAs(path);
                }
                return View(cardKey);
            }

            UpdateRequestClearances(form["Clearances"].Split(','), cardKey.Request);

            if (ModelState.IsValid)
            {
                db.Entry(cardKey).State = EntityState.Modified;
                db.SaveChanges();
                return RedirectToAction("Index");
            }
            ViewBag.DepartmentID = new SelectList(db.Departments, "DepartmentID", "DepartmentName", cardKey.DepartmentID);
            ViewBag.DivisionID = new SelectList(db.Divisions, "DivisionID", "DivisionName", cardKey.DivisionID);
            ViewBag.ProgramID = new SelectList(db.Programs, "ProgramID", "ProgramName", cardKey.ProgramID);
            return View(cardKey);
        }

查看:(我省略了许多不相关的代码）

@using (Html.BeginForm("Edit","CardKeys",FormMethod.Post,new { enctype="multipart/form-data"}))
{
    @Html.AntiForgeryToken()

    <div class="form-horizontal">
        <h4>CardKey</h4>
        <hr />
<div class="form-group">
            @Html.Label("File Upload", new { @class="control-label col-md-4"})
            <div class="col-md-8">
                <input type="file" multiple="multiple" id="files" />
                <input type="submit" name="addFileButton" id="btnAddFile" value="add files" />
            </div>
        </div>        
        <div class="form-group">
            <div class="col-md-offset-2 col-md-10">
                <input type="submit" value="Save" class="btn btn-default" />
            </div>
        </div>
    </div>

我以管理员身份打开visual studio，以确保这不是权限问题。