private function _checkFileRelationExists(Model $model, $relationName)
    {
        if ((!method_exists($model, $relationName) && !$model->{$relationName}() instanceof Relation)) {
            if (Lang::has('resource-controller.filerelationinexistent')) {
                $message = trans('resource-controller.filerelationinexistent', ['relationName' => $relationName]);
            } else {
                $message = "Request file '{$relationName}' is not named after an existent relation.";
            }
            throw new UploadException($message);
        }
    }