public function get_option($config = '') {
        if ($config === 'dropbox_key') {
            return trim(get_config('dropbox', 'dropbox_key'));
        } else if ($config === 'dropbox_secret') {
            return trim(get_config('dropbox', 'dropbox_secret'));
        } else if ($config === 'dropbox_cachelimit') {
            return $this->max_cache_bytes();
        } else {
            $options = parent::get_option();
            $options['dropbox_key'] = trim(get_config('dropbox', 'dropbox_key'));
            $options['dropbox_secret'] = trim(get_config('dropbox', 'dropbox_secret'));
            $options['dropbox_cachelimit'] = $this->max_cache_bytes();
        }

        return $options;
    }