add_filter('get_post_metadata', function($value, $object_id, $meta_key, $single) {
    if ($meta_key !== '_thumbnail_id' || $value) {
        return $value;
    }
     preg_match('~<img[^>]+wp-image-(\\d+)~', get_post_field('post_content', $object_id), $matches);
    if ($matches) {
        return $matches[1];
    }
    return $value;
}, 10, 4);