protected function notFound()
    {
        // Set the default not found response
        $this->response->setContent('File not found');
        $this->response->headers->set('content-type', 'text/html');
        $this->response->setStatusCode(Response::HTTP_NOT_FOUND);

        // Execute the user defined notFound callback
        if (is_callable($this->notFound)) {
            $this->response = call_user_func($this->notFound, $this->response);
        }
    }