Unit test for a controller's action method with HttpPostedFileBase parameter

Question

I have an action method as below

    [HttpPost]
    public ActionResult Index(HttpPostedFileBase file)
    {
        //do something
        return View();
    }

and front end is

 @using (Html.BeginForm("Index", "Home", FormMethod.Post, new { enctype = "multipart/form-data" }))
           {
                <input type="file" name="file" id="filesUpload" />
                <input type="submit" value="Upload File" />
           }

Well, my code just work fine. But I want to write an unit test for the action method void Index (HttpPostedFileBase file). Where I want to pass the file as the way we got from UI form post. How can I do it.