rss enclosure

The standard WordPress RSS-feeds don’t include posts featured image. Below code adds the medium-format thumbnail to each item in a RSS2 standards-compliant manner by inserting it as an enclosure.

add_action('rss2_item', 'add_enclosure_thumb');
function add_enclosure_thumb() {
  global $post;
  if(has_post_thumbnail($post->ID)) {
    $thumbUrl = get_the_post_thumbnail_url($post->ID,"medium");
    if ((substr($thumbUrl, -4) === "jpeg") || (substr($thumbUrl, -3) === "jpg")) {
      $mimeType="image/jpeg";
    } else if (substr($thumbUrl, -3) === "png") {
      $mimeType="image/png";
    } else if (substr($thumbUrl, -3) === "gif") {
      $mimeType="image/gif";
    } else {
      $mimeType="image/unkown";
    }
    $thumbSize = filesize(WP_CONTENT_DIR.str_replace(WP_CONTENT_URL,'',$thumbUrl));
    echo "<enclosure url=\"".$thumbUrl."\" size=\"".$thumbSize."\" type=\"".$mimeType."\" />\n";
  }
}

A more advanced & flexible approach would be to add support for the media RSS namespace, but the above suffices for the purpose I have in mind.