Any media file not on development server will be loaded from live server.

dev-server-media-files.php

<?php add_action('init', 'my_replace_image_urls' );
function my_replace_image_urls() {
    if ( defined('WP_SITEURL') && defined('LIVE_SITEURL') ) {
        if ( WP_SITEURL != LIVE_SITEURL ){
            add_filter('wp_get_attachment_url', 'my_wp_get_attachment_url', 10, 2 );
        }
    }
}

function my_wp_get_attachment_url( $url, $post_id) {
    if ( $file = get_post_meta( $post_id, '_wp_attached_file', true) ) {
        if ( ($uploads = wp_upload_dir()) && false === $uploads['error'] ) {
            if ( file_exists( $uploads['basedir'] .'/'. $file ) ) {
                return $url;
            }
        }
    }
    return str_replace( WP_SITEURL, LIVE_SITEURL, $url );
}

?>