Featured image rss doesn't work

I add the code below to Newsmag/functions.php file but the RSS doesn’t show thumbnail.
// THIS INCLUDES THE THUMBNAIL IN OUR RSS FEED
function insertThumbnailRSS($content) {
global $post;
if ( has_post_thumbnail( $post->ID ) ){
$content = '' . get_the_post_thumbnail( $post->ID, 'thumbnail', array('style' => 'margin-bottom:10px;float:right;') ) . '' . $content;
}
return $content;
}

add_filter('the_excerpt_rss', 'insertThumbnailRSS');
add_filter('the_content_feed', 'insertThumbnailRSS');


Thank for helping. Hope to hear respond soon. Thank again

Here is the link http://nhipsongtre360.com/bai-thuoc-ki-dieu-diet-tan-goc-benh-thoai-hoa-cot-song-va-dia-dem/
Sorry forgot the link (The RSS is in the widget sidebar). It only showing the title.

Thank Lucian

Hey Lucian,

Could give me some advice on this topic. I’ve implemented the code into functions.php, a thumbnail of the featured image is visible. Is it possible to add a full image to the RSS-feed instead of the thumbnail?

Thanks in advance!
Tim