var mt = db.MtoTransparencia.Find(id);

if (mt == null)
{
    return HttpNotFound();
}

var adjunto = new MtoTransparenciaAdjunto
{
    MtoTransparenciaId = mt.MtoTransparenciaId,
};
return View(adjunto);

@model WebDBT.Models.MtoTransparenciaAdjunto
@{

    Layout = null;
}

@using (@Html.BeginForm("UploadAdjunto", "MtoTransparencia", FormMethod.Post, new { @class = ".FrmUpload" }))
{
    @Html.AntiForgeryToken();
    @Html.HiddenFor(model => model.MtoTransparenciaId);


<div class="form-horizontal">
    <hr />
    @Html.ValidationSummary(true, "", new { @class = "text-danger" })
    <div class="form-group">
        @Html.LabelFor(model => model.Ruta, htmlAttributes: new { @class = "control-label col-md-2" })
        <div class="col-md-10">
            @Html.TextBoxFor(model => model.Ruta, "", new { @type = "file", @multiple = "multiple", @name = "file" })
            @Html.ValidationMessageFor(model => model.Ruta, "", new { @class = "text-danger" })
        </div>
    </div>
    <div class="form-group">
        <div class="col-md-offset-2 col-md-10">
            <input type="submit" value="Upload" class="btn btn-primary" />
        </div>
    </div>
    <div class="form-group">
        <div class="col-md-offset-2 col-md-10 text-success">
            @ViewBag.FileStatus
        </div>
    </div>
</div>



@section Scripts {
    @Scripts.Render("~/bundles/jqueryval")
    @Scripts.Render("~/bundles/WEBDBT")
    @Scripts.Render("~/Scripts/WebScript/MtoTransparencia.js")
}

[ValidateAntiForgeryToken, HttpPost]
        public ActionResult UploadAdjunto(HttpPostedFileBase File,MtoTransparenciaAdjunto model)
        {
            var user = db.MtoUsuarios.Where(u => u.UserName == User.Identity.Name).FirstOrDefault();
            if (user == null)
            {
                RedirectToAction("Index", "Home");
            }

            File = File ?? Request.Files["Ruta"];

            if (File.ContentLength != 0)
            {
                string extension = System.IO.Path.GetExtension(File.FileName).ToLower();

                string[] validFileTypes = { ".xls", ".xlsx", ".csv" };
            }
            return View();

            }

File = File ?? Request.Files["Ruta"];

System.NullReferenceException: 'Referencia a objeto no establecida como instancia de un objeto.'