How to add an enclosure to a WordPress RSS feed using the featured image of the post - add to functions.php

Wordpress RSS 2.0 featured image enclosure.php

<?php
/**
 * Wordpress RSS 2.0 image enclosure.
 *
 * @param  WP_Query $query The current WP_Query instance.
 * @return WP_Query
 */
add_filter( 'pre_get_posts', 'feed_filter' );
function feed_filter( $query ) {
    if ( $query->is_feed ) {
        add_filter( 'rss2_item', 'feed_content_filter');
    }

    return $query;
}

/**
 * How to add an enclosure to a wordpress RSS feed using the featured image of the
 * post [with its actual length attribute value].
 *
 * @param  mixed $item Current feed item.
 * @return mixed       Feed item.
 */
function feed_content_filter( $item ) {

    global $post;

    if(!has_post_thumbnail($post->ID)) return;
    $thumbnail_id = get_post_thumbnail_id( $post->ID );
    if(empty($thumbnail_id)) return;
    
    $image_full = wp_get_attachment_image_src( $thumbnail_id, 'full' );
    $image_medium = wp_get_attachment_image_src( $thumbnail_id, 'medium' );
    $image_thumb = wp_get_attachment_image_src( $thumbnail_id, 'thumbnail' );
    $image_mime = get_post_mime_type($thumbnail_id);

    echo '<enclosure url="' . $image_thumb[0] . '" length="' . filesize( $image_thumb[0] ) . '" type="' . $image_mime . '"/>';

    return $item;

}