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

add_filter( 'pre_get_posts', 'feed_filter' );
function feed_filter( $query ) {
    if ( $query->is_feed ) {
        add_filter( 'rss2_item', 'feed_content_filter');
    }
    return $query;
}

function feed_content_filter() {
    global $post;
    if( has_post_thumbnail( $post->ID ) ) {
        $image_id = get_post_thumbnail_id( $post->ID );
        $url = wp_get_attachment_image_url( $image_id, 'large' );
        $file_type = wp_check_filetype( basename( $url ) );
        $image_type = $file_type['type'];
        $attach = wp_get_attachment_metadata( $image_id );
        $upload_dir = wp_upload_dir();
        $length = 0;
        $file = $upload_dir['basedir'] . '/' . $attach['file'];
        if ( file_exists( $file ) ) {
            $length = filesize( $file );
        }
        echo "<enclosure url=\"{$url}\" length=\"$length\" type=\"{$image_type}\" />";
    }

}