How to Show Post Thumbnails in RSS Feed in WordPress?

function wps_feed_post_thumbnail($content) {
        global $post;
        if(has_post_thumbnail($post->ID)) {
        $content = '' . $content;
        }
        return $content;
        }
        add_filter('the_excerpt_rss', 'wps_feed_post_thumbnail');
        add_filter('the_content_feed', 'wps_feed_post_thumbnail');