reverted param naming to 'attribute'

Sample upload action:

```
    /**
     * Action for file uploads via sir-trevor image block from SirTrevorWidget (input widget)
     *
     * @param $root relative base folder
     */
    public function actionSirTrevorUpload($root)
    {
        $upload         = UploadedFile::getInstanceByName('attachment[file]');

        $model          = new File();
        $model->name_id = Inflector::slug(str_replace($upload->extension, '', $upload->name));
        $model->name_id = $this->checkFileNameNotExist($model);
        $model->path    = 'images/' . $root . '/' . $model->name_id . '.' . $upload->extension;
        $savePath       = \Yii::getAlias('@backend/web') . '/' . $model->path;

        if (!is_file($savePath) && is_dir(dirname($savePath))) {
            if ($upload->saveAs($model->path)) {
                $model->mime_type = FileHelper::getMimeType($model->path);
                $model->file_size = $upload->size;
                if ($model->save()) {
                    $items = $this->getItems($this->rootPath);
                    list($width, $height, $type, $attr) = getimagesize($model->path);
                    echo Json::encode(['file' => ['url' => $model->path]]);
                    return;
                } else {
                    echo "Database record could not be saved.";
                }
            } else {
                echo "File could not be saved.";
            }
        } else {
            echo "File exists or root folder '{$savePath}' not found.";
        }
    }
````