Add post thumbnails to RSS feed

post_thumbnail_rss.php

// Add post thumbnails to RSS feed
function rss_post_thumbnail($content) {
  global $post;
  if(has_post_thumbnail($post->ID)) {
    $content = '<p>' . get_the_post_thumbnail($post->ID) . '</p>' . get_the_content();
  }
  return $content;
}
add_filter('the_excerpt_rss', 'rss_post_thumbnail');
add_filter('the_content_feed', 'rss_post_thumbnail');