public function urlExtension()
    {
        if ($this->home()) {
            return '';
        }

        // if not set in the page get the value from system config
        if (null === $this->url_extension) {
            $this->url_extension = Grav::instance()['config']->get('system.pages.append_url_extension', '');
        }

        return $this->url_extension;
    }