@using (Ajax.BeginForm("Upload", "Attachment", FormMethod.Post, 
new AjaxOptions
                            {
                                HttpMethod = "POST",
                            },
                            new
                            {
                                encType = "multipart/form-data",
                                id = "attach-form"
                            }))
{
    @Html.AntiForgeryToken()

    @Html.TextBoxFor(m => m.FirstName })
    
    <input type="file" name="Files" data-buttonText="انتخاب تصویر">
   
    <button type="submit">ارسال</button>
}

        var formData = new FormData();

        $('form').submit(function() {
            
            var action = $(this).attr('action');
            var formData = new FormData($(this).get(0));

            $.ajax({
                type: "POST",
                dataType: "json",
                url: action,
                data: formData,
                processData: false,
                contentType: false,
                success: function(data) {
                    //...
                }
                success: function(data) {
                     //...
                }
            });

            return false;
        });

public class MyModel
    {
        public string FirstName{ get; set; }

        public IEnumerable<HttpPostedFileBase> Files { get; set; }
    }  

[AjaxOnly]
[HttpPost]
[ValidateAntiForgeryToken]
public ActionResult Upload(MyModel model)
{
     if (!ModelState.IsValid)
         return //....

     if(model.Files != null)
         foreach (var file in model.Files)
             if (file  != null && file.ContentLength > 0)
             {
                 // ....
             }
}