WP_Import::is_valid_meta_key PHP Method

        function is_valid_meta_key($key)
        {
            // skip attachment metadata since we'll regenerate it from scratch
            // skip _edit_lock as not relevant for import
            if (in_array($key, array('_wp_attached_file', '_wp_attachment_metadata', '_edit_lock'))) {
                return false;
            }
            return $key;
        }