protected function registerPreviewSwiftMailer()
    {
        $this->app->singleton('swift.mailer', function($app) {
            return new Swift_Mailer(
                new PreviewTransport(
                    $app->make('Illuminate\Filesystem\Filesystem'),
                    $app['config']['mailpreview.path'],
                    $app['config']['mailpreview.maximum_lifetime']
                )
            );
        });
    }