<form method="post" action="/" id="editForm" name="editForm" enctype = "multipart/form-data">

       <input type="file" value="Выбрать файл..." name="btnFile" id="btnFile" />
       <input type="submit" value="Сохранить" id="btnSubmit"/> 

       </form>

----------------------------------------------------------------------------

 $("#editForm").submit(function() {

            $.ajax({
                method: "POST",
                url: "@Url.Action("SaveProduct", "Home")/?id=@idgr&mTab=@mtab",
                data: $("#editForm").serialize(),
                dataType: "JSON",
                success: function () {
                    alert("ee");
                },
                error:  function(xhr, str){
                    alert('Возникла ошибка: ' + xhr.responseCode);
                }
            });
        });




--------------------------------------------------------------------------

  [HttpPost]
        public JsonResult SaveProduct(int? id, string mTab, FormCollection frm, HttpPostedFileBase btnFile)
        {
            switch (mTab)
            {
                case "equip_si":
                    SaveProductSI(frm, id, btnFile);
                    break;
                case "equip_io":
                    SaveProductIO(frm, id, btnFile);
                    break;
                case "equip_sk":
                    SaveProductSK(frm, id, btnFile);
                    break;

            }
            return Json(new { success = true, error = "", msg = "Все ОК" });
        }