ASP.net MVC上载图像NullReferenceException

我已经实现了简单的创建方法，将数据放入数据库，它工作得很好。我决定添加图像上传，但我一直得到NullReferenceException(文件为空)，我不知道为什么。我希望你能帮助我!

下面是我的代码:

[Authorize]
    public ActionResult Create()
    {
        ViewBag.CategoryId = new SelectList(_categoryRepository.GetAllCategories().AsEnumerable(), "CategoryId",
                                            "Name");
        return View();
    }
    //
    // POST: /Advert/Create
    [HttpPost, Authorize]
    public ActionResult Create(CompetitionDTO competitionDTO, HttpPostedFileBase file)
    {
        if (file.ContentLength > 0)
        {
            string fileName = Path.GetFileName(file.FileName);
            string fileExtension = Path.GetExtension(fileName);
            if ((fileExtension == ".jpg") || (fileExtension == ".png"))
            {
                string path = Path.Combine(Server.MapPath("~/Uploads/Images"), fileName);
                file.SaveAs(path);
                competitionDTO.ImageURL = path;
            }
        }
        if (ModelState.IsValid)
        {
            _competitionRepository.AddCompetition(competitionDTO, WebSecurity.CurrentUserId);

            return RedirectToAction("Index");
        }
        ViewBag.CompetitionId = new SelectList(_competitionRepository.GetAllCompetitions().AsEnumerable(),
                                               "CompetitionId", "Name");

        return View(competitionDTO);
    }
}

和View

<div>
        @using (Html.BeginForm("Create", "Competition", FormMethod.Post, new
            {
                enctype = "multipart/form-data"
                , id = "parentForm"
            }))
        {
            <input type="file" name="file" id="file"/>
            <input type="submit" value="Create" />
        }
    </div>

编辑

如果我没说清楚的话:

我知道如何插入数据到数据库我知道如何上传文件，我希望当我点击提交时这两件事同时发生