HttpPostedFileBase为空

前台：
<% using (Html.BeginForm("UpPhoto", "Albumn", FormMethod.Post, new { enctype = "multipart/form-data" }))
{%>

   选择相册<%:Html.DropDownList("Name") %>
    <%: Html.ActionLink("创建新相册", "Create") %>

              <input type="file" name="head" class="filebtn"  />
                <input type="submit" class="upimgbtn" value="上传相片""/>   

<%} %>
    后台： public ActionResult UpPhoto(HttpPostedFileBase filedate)
    {

            }

action 标记 [HttpPost]