Si no me refierojquery.unobtrusive-ajax.js Puedo obtener el archivo adjunto en Post. Si lo remito me está dando nulo.

<script src="~/Scripts/jquery.unobtrusive-ajax.js"></script>


@using (Ajax.BeginForm("Index", "ContactSubmission", new AjaxOptions{ InsertionMode = InsertionMode.Replace, HttpMethod = "POST", OnSuccess = "updateSuccess" },
     new { enctype = "multipart/form-data",@class = "form-horizontal", role = "form" }))
      {
               ///code here

}

[HttpPost]
public JsonResult Index(Contact contact)
{
    if (ModelState.IsValid)
    {
       if (contact != null)
       {
         string attachment = string.Empty;
         // HttpPostedFileBase Attachment
         if (contact.Attachment != null) attachment = SaveFile(contact.Attachment); 
                ......

¿Cómo manejar esto?