public function send_thumbnail($source) {
        $content = $this->dropbox->get_thumbnail($source);

        // Set 30 days lifetime for the image.
        // If the image is changed in dropbox it will have different revision number and URL will be different.
        // It is completely safe to cache the thumbnail in the browser for a long time.
        send_file($content, basename($source), 30 * DAYSECS, 0, true);
    }