Upload file is null in mvc controller?

Question

In controller using HttpPostedFileBase to get file upload

 [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create([Bind(Include = "MaSP,TenSP,DonViTinh,DonGia,MaNCC,SoLuong,Anh,MoTaSP")] SanPham sanPham, HttpPostedFileBase file)
    {
        if (ModelState.IsValid)
        {
            if (file.FileName != null)
            {
                file.SaveAs(HttpContext.Server.MapPath("~/Images/")
                            + file.FileName);
            }
            db.SanPhams.Add(sanPham);
            db.SaveChanges();
            return RedirectToAction("Index");
        }

        ViewBag.MaNCC = new SelectList(db.NhaCungCaps, "MaNCC", "TenNCC", sanPham.MaNCC);
        return View(sanPham);
    }

In Create.cshtml

    @using (Html.BeginForm()) 
{
    @Html.AntiForgeryToken()
.....
<div class="form-group">
            @Html.LabelFor(model => model.Anh, htmlAttributes: new { @class = "control-label col-md-2" })
            <div class="col-md-10">
                <input id="Anh" title="Upload hình sản phẩm" 
                       type="file" name="file" class="form-control"/>
                @Html.ValidationMessageFor(model => model.Anh, "", new { @class = "text-danger" })
            </div>
        </div>

I clicked a button - viewing this function in controller then it got null HttpPostedFileBase file -> file got null