public function actionUpdateThumbnail()
 {
     $form = new FileUploadModel();
     $form->setScenario(FileUploadModel::SCENARIO_IMAGE);
     if (\Yii::$app->request->isPost) {
         $form->uploadedFile = UploadedFile::getInstance($form, 'uploadedFile');
     }
     if (!$form->validate()) {
         $ex = new ValidationException();
         $ex->setData($form->errors);
         throw $ex;
     } else {
         $form->uploadedFile->saveAs('assets/thumbnail.' . $form->uploadedFile->extension);
     }
     return [];
 }