WP Json Endpoint with file extension .json

Question

I want my wp json endpoint to end with .json file extension. I have tried doing regular expression but it didn't work. What did i do wrong? Or is it even possible?

Intention to have url as: domain.com/wp-json/feed_mmagic/v1/article/123.json

add_action('rest_api_init', function () {
  register_rest_route( 'feed_magic/v1', 'article/(?P<feed_id>\d+)(.json)',array(
                'methods'  => 'GET',
                'callback' => 'json_get_article'
      ));
});


function json_get_article($request){
        header('Content-Type: application/json; charset=utf-8');
    global $wp_query;

    $feedid = $request['feed_id'];
    $content = get_post( $feedid );
    $response = new WP_REST_Response($content);
return $response;
}