How can I reference external attachments without breaking core WordPress files?

Pay attention to the comments along the snippet.

<?php
/**
 * Filters the attachment URL.
 *
 * @param   string $url    URL for the given attachment.
 * @param   int    $att_id Attachment post ID.
 *
 * @return  string  $url Custom URL for the given attachment.
 */
function wp_get_attachment_url_callback( $url, $att_id ) {
    // Instead of keeping full path we actually need just 'wp-content/uploads'.
    // And we do this the right way, dynamically, calling functions and constants.
    $uploads_dir         = wp_get_upload_dir()['basedir'];
    $partial_uploads_dir = str_replace( ABSPATH, '', $uploads_dir );

    // Check if attachment file is in WordPress uploads directory.
    if ( strpos( $url, $partial_uploads_dir ) === false ) {
        return $url;
    }

    // Just for reference, until now, the $url is still something like:
    // http://example.com/wp-content/uploads/2019/03/image.jpg

    /**
     * @TODO    Define the right CDN URL here.
     */
    $new_site_url="http://cdn-domain.com";
    $pattern      = get_site_url();
    $url          = preg_replace( "#$pattern#", $new_site_url, $url );

    // Again, just for reference, now the $url looks like:
    // http://cdn-domain.com/wp-content/uploads/2019/03/image.jpg

    return $url;
}

add_filter( 'wp_get_attachment_url', 'wp_get_attachment_url_callback', 999, 2 );

I hope it enlightens you so you can achieve your goal.

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