DefaultFormatTestController.php

<?php

namespace Drupal\default_format_test;

use Drupal\Core\Cache\CacheableResponse;
use Symfony\Component\HttpFoundation\Request;
class DefaultFormatTestController {
    public function content(Request $request) {
        $format = $request->getRequestFormat();
        return new CacheableResponse('format:' . $format, 200, [
            'Content-Type' => $request->getMimeType($format),
        ]);
    }

}