Example WordPress plugin to help simplify attachment URLs on multisite sites with mapped domains

multisite_attachment_url.php

<?php
/**
 * Plugin Name: Multisite Domain Mapping Attachment URL Fixes
 * Description: Update attachment URLs to use mapped domain and remove mention of "sites" path.
 * Author: Chris Hardie
 *
 * Rewrite attachment URLs (and related srcset URLs) to the non-multisite, mapped domain version if a domain is mapped
 * Requires that the related nginx config that maps the non-multisite URL to the multisite URL be in place
**/
add_filter( 'wp_get_attachment_url', 'jch_attachment_url_with_domain_mapping' );

function jch_attachment_url_with_domain_mapping( $attachment_url ) {

    global $wpdb;

    // Comes from https://wordpress.org/plugins/wordpress-mu-domain-mapping/
    if ( function_exists( 'get_original_url' ) && function_exists( 'domain_mapping_siteurl' ) ) {
        $orig_url = get_original_url( 'siteurl' );

        $mapped_url = domain_mapping_siteurl( 'NA' );

        if ( 'NA' !== $mapped_url ) {
            if ( ! empty( $orig_url ) && ( $orig_url !== $mapped_url ) ) {
                $attachment_url = str_replace( $orig_url, $mapped_url, $attachment_url );
            }
            $attachment_url = preg_replace( '!wp-content/uploads/sites/\d+/!', 'wp-content/uploads/', $attachment_url );
        }
    }

    return $attachment_url;

}

add_filter( 'wp_calculate_image_srcset', 'jch_srscet_urls_with_domain_mapping' );

function jch_srscet_urls_with_domain_mapping( $sources ) {

    if ( function_exists( 'domain_mapping_siteurl' ) ) {
        $domain_mapped_url = domain_mapping_siteurl( 'NA' );

        if ( 'NA' !== $domain_mapped_url ) {
            foreach ( $sources as $source ) {
                if ( ! empty( $source['url'] ) && ! empty( $source['value'] ) ) {
                    $sources[ $source[ 'value' ] ][ 'url' ] = preg_replace( '!wp-content/uploads/sites/\d+/!', 'wp-content/uploads/', $sources[ $source[ 'value' ] ][ 'url' ] );
                }
            }
        }
    }

    return $sources;
}