public function addHeader(EventInterface $event) {
        $request = $event->getRequest();
        $response = $event->getResponse();

        $user = $request->getUser();
        $imageIdentifier = $response->getModel()->getImageIdentifier();

        $response->headers->set(
            $this->header,
            [
                'imbo;image;' . $user . ';' . $imageIdentifier,
                'imbo;user;' . $user,
            ]
        );
    }