[Question]Asset Manager Upload in ASP.Net

I want to save file when i upload in dropzone, to do in init

        assetManager: {
            upload: '/SuperAdmin/UploadAsset',
            uploadFile: function (e) {
                var files = e.dataTransfer ? e.dataTransfer.files : e.target.files;
                // ...send somewhere
                console.log(files);

                var formData = new FormData();

                for (var i in files) {
                    formData.append('file-' + i, files[i])
                }

                $.ajax({
                    url: '/SuperAdmin/UploadAsset',
                    type: 'POST',
                    data: formData,
                    contentType: false,
                    crossDomain: true,
                    mimeType: "multipart/form-data",
                    processData: false,
                    success: function (result) {
                        console.log(result);

                    }
                });
            }

And in my Controler ASP.Net SuperAdmin :

        public JsonResult UploadAsset(string formdata)
        {
           return Json(object(?), JsonRequestBehavior.AllowGet);
        }

But formdata is always null ???

What i wrong ? (i'm beginer why JSon)

Merci beaucoup.