//
//JS
//
function testDownload()
    {
        $.ajax({
            cache: false,
            type: "POST",
            url: '@Url.Action("myTest", "Home")',
            success: function (result) {
                //window.location = result.location;
                alert(result.location);
            },
            error: function () {
                alert("Failed.");
            }
        });
    }

//
//C#
//
[HttpPost]
        public ActionResult myProva()
        {
            byte[] fileBytes = System.IO.File.ReadAllBytes(@"C:\Users\MyUser\Desktop\MyFile.txt");
            string fileName = "MyFile.ext";
            Response.Headers.Add("Content-Disposition", $"attachment; filename=\"{fileName}\"");
            return File(fileBytes, "application/txt", fileName);
        }