public function setExposedHeaders(EventInterface $event) {
        // If this request was disallowed, don't expose any headers
        if (!$this->requestAllowed) {
            return;
        }

        $headers = [
            // The ResponseSender-listener will add this header and send the response,
            // so we have no way to pick it up - instead we'll always whitelist it
            'X-Imbo-ImageIdentifier'
        ];

        foreach ($event->getResponse()->headers as $header => $value) {
            if (strpos($header, 'x-imbo') === 0) {
                $headers[] = implode('-', array_map('ucfirst', explode('-', $header)));;
            }
        }

        $event->getResponse()->headers->add([
            'Access-Control-Expose-Headers' => implode(', ', $headers)
        ]);
    }