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function add_feed_post_thumbnail($content) { 

    global $post;

    if(has_post_thumbnail($post->ID)) { 

        $content = get_the_post_thumbnail($post->ID, 'thumbnail') . $content; 

        } 

        return $content; 

        } 

add_filter('the_excerpt_rss', 'add_feed_post_thumbnail');

add_filter('the_content_feed', 'add_feed_post_thumbnail');