You should add enctype as multipart/form-data
@using (Html.BeginForm(nameof(ServiceCallController.AddMessage), "ServiceCall", FormMethod.Post, new { enctype = "multipart/form-data" }))
And the action method:
[HttpPost]
public ActionResult AddMessage(int id, string newMessage, HttpPostedFileBase myFile)
{
if (myFile == null) throw new ArgumentNullException(nameof(myFile));//This should not happen
return RedirectToAction(nameof(MyView), new { id });
}
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