// Add the upload progress callback.
$element['upload_button']['#ajax']['progress']['url'] = Url::fromRoute('file.ajax_progress', ['key' => $upload_progress_key]);
+
+ // Set a custom submit event so we can modify the upload progress
+ // identifier element before the form gets submitted.
+ $element['upload_button']['#ajax']['event'] = 'fileUpload';
}
// The file upload field itself.