Ajax FileUpload & JQuery formData in ASP.NET MVC

I have some problem with posting formData to server side action method. Because ajax call doesn't send files to server, I have to add file uploader data to formData manually like this:

var formData = new FormData();
formData.append("imageFile", $("#imageFile").file);
formData.append("coverFile", $("#coverFile").file);

I wrote jQuery function that need to post form data to server using ajax call. It's works but posted formData in server side is always null!

this is my script:

    <script>
        function SubmitButtonOnclick()
        {    
            var formData = new FormData();
            formData.append("imageFile", $("#imageFile").file);
            formData.append("coverFile", $("#coverFile").file);

            $.ajax({
                type: "POST",
                url: '@Url.Action("EditProfile", "Profile")',
                data: formData,
                dataType: 'json',
                contentType: "multipart/form-data",
                processData: false,
                success: function (response) {
                    $('#GeneralSection').html(response.responseText);
                },
                error: function (error) {
                    $('#GeneralSection').html(error.responseText);
                }
            });
        }
    </script>

Edit 1: This is the action method in controller:

        public ActionResult EditProfile(ProfileGeneralDescription editedModel,
                                HttpPostedFileBase imageFile,
                                HttpPostedFileBase coverFile,
                                string postOwnerUser)
        {
            try
            {
                if (postOwnerUser == User.Identity.Name)
                {
                // edit codes...    
                    var result = GetProfileGeneralDescription(postOwnerUser);
                    return PartialView("_GeneralProfile", result);
                }
                else
                {
                    var error = new HandleErrorInfo(
                    new Exception("Access Denied!"),
                    "Profile",
                    EditProfile
                    return PartialView("~/Views/Shared/Error.cshtml", error);
                }
            }
            catch (Exception ex)
            {
                var error = new HandleErrorInfo(ex, "Profile", EditProfile
                return PartialView("~/Views/Shared/Error.cshtml", error);
            }
        }

Edit 2: Cshtml view file content:

@model Website.Models.ViewModel.Profile

    @using (Ajax.BeginForm("EditProfile", "Profile", new { postOwnerUser = User.Identity.Name }, new AjaxOptions()
    {
        HttpMethod = "POST",
        InsertionMode = InsertionMode.Replace,
        UpdateTargetId = "GeneralSection"
    }, new { enctype = "multipart/form-data" }))
    {

        <div>
             <button id="btnSubmit" type="button" onclick="SubmitButtonOnclick()">Submit</button>
        </div>

        <input type="hidden" name="username" id="username" value="@Model.Username"/>

        <fieldset>
            <legend>Edit Photos</legend>
            <div>
                Select profile picture:
                <input id="imageFile" type="file" name="imageFile" accept="image/png, image/jpeg" />
                @Html.CheckBoxFor(modelItem => modelItem.DefaultCover)<span>Remove profile photo</span>
            </div>
            <div>
                Select cover picture:
                <input id="coverFile" type="file" name="coverFile" accept="image/png, image/jpeg" />
                @Html.CheckBoxFor(modelItem => modelItem.DefaultCover)<span>RemoveCover</span>
            </div>
        </fieldset>
    }

Any tips, link or code example would be useful.
Thank you in advance!