protected function get_thumbnail_url($entry) {
        if ($this->dropbox->supports_thumbnail($entry)) {
            $thumburl = new moodle_url('/repository/dropbox/thumbnail.php', [
                // The id field in dropbox is unique - no need to specify a revision.
                'source'    => $entry->id,
                'path'      => $entry->path_lower,

                'repo_id'   => $this->id,
                'ctx_id'    => $this->context->id,
            ]);
            return $thumburl->out(false);
        }

        return '';
    }