private static function is_internal_content($url)
{
// using content_url() instead of site_url or home_url is IMPORTANT
// otherwise you run into errors with sites that:
// 1. use WPML plugin
// 2. or redefine content directory
$is_content_url = strstr($url, content_url());
// this case covers when the upload directory has been redefined
$upload_dir = wp_upload_dir();
$is_upload_url = strstr($url, $upload_dir['baseurl']);
return $is_content_url || $is_upload_url;
}