How to add an enclosure to a wordpress RSS feed using the first image of the post [with its actual length attribute value] - add to functions.php

Wordpress RSS 2.0 image enclosure [COMPLETE with IMG filesize included].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 first 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;

    $args = array(
        'order'          => 'ASC',
        'post_type'      => 'attachment',
        'post_parent'    => $post->ID,
        'post_mime_type' => 'image',
        'post_status'    => null,
        'numberposts'    => 1,
    );

    $attachments = get_posts( $args );

    if ( $attachments ) {
        foreach ( $attachments as $attachment ) {
            $image = wp_get_attachment_image_src( $attachment->ID, 'large' );
            $mime  = get_post_mime_type( $attachment->ID );
        }
    }

    if ( $image ) {
        echo '<enclosure url="' . $image[0] . '" length="' . remote_file_size( $image[0] ) . '" type="' . $mime . '"/>';
    }

    return $item;

}

/**
 * Get the remote file size.
 *
 * @see {@link http://www.w3bees.com/2013/03/get-remote-file-size-using-php.html}
 * @author Resalat Haque
 *
 * @param  string $url Remote file URL.
 * @return int         File size in bytes.
 */
function remote_file_size( $url ) {
    // Get all header information
    $data = get_headers($url, true);
    // Look up validity
    if ( isset( $data['Content-Length'] ) ) {
        // Return file size
        return (int) $data['Content-Length'];
    }
}