title: Model is null when form submitted
body: <p>When I hit submit, the <code>file</code> parameter is null.</p>
  
  <pre><code>public ActionResult Create()
  {
    return View(new FileViewModel());
  }
  
  [HttpPost]    
  [InitializeBlobHelper]
  public ActionResult Create(FileViewModel file)
  {
    if (ModelState.IsValid)
    {
       //upload file
    }
    else
      return View(file);
  }
  
  public class FileViewModel
  {
    internal const string UploadingUserNameKey = "UserName";
    internal const string FileNameKey = "FileName";
  
    internal const string Folder = "files";
  
    private readonly Guid guid = Guid.NewGuid();
  
    public string FileName
    {
      get
      {
        if (File == null)
          return null;
        var folder = Folder;
        return string.Format("{0}/{1}{2}", folder, guid, Path.GetExtension(File.FileName)).ToLowerInvariant();
      }
    }
  
    [RequiredValue]
    public HttpPostedFileBase File { get; set; }
  }
  </code></pre>
  
  <p>Here is the cshtml:</p>
  
  <pre><code>@model MyProject.Controllers.Admin.FileViewModel
  
  @{
    ViewBag.Title = "Create";
    Layout = "~/Views/Shared/_BackOfficeLayout.cshtml";
  }
  
  @using (Html.BeginForm("Create", "Files", FormMethod.Post, new { enctype = "multipart/form-data" }))
  {
    &lt;fieldset&gt;
      &lt;legend&gt;Create&lt;/legend&gt;
  
      &lt;div class="editor-label"&gt;
        @Html.LabelFor(model =&gt; model.File)
      &lt;/div&gt;
      &lt;div class="editor-field"&gt;
        @Html.TextBoxFor(model =&gt; model.File, new { type = "file" })
        @Html.ValidationMessageFor(model =&gt; model.File)
      &lt;/div&gt;
  
      &lt;p&gt;
        &lt;input type="submit" value="Create" /&gt;
      &lt;/p&gt;
    &lt;/fieldset&gt;
  }
  
  &lt;div&gt;
    @Html.ActionLink("Back to List", "Index")
  &lt;/div&gt;
  </code></pre>