public function setDispositionParameter($label, $data)
    {
        $cd = $this->_headers['content-disposition'];

        if (is_null($data)) {
            unset($cd[$label]);
        } elseif (strlen($data)) {
            $cd[$label] = $data;

            if (strcasecmp($label, 'size') === 0) {
                // RFC 2183 [2.7] - size parameter
                $this->_bytes = $cd[$label];
            } elseif ((strcasecmp($label, 'filename') === 0) &&
                      !strlen($cd->value)) {
                /* Set part to attachment if not already explicitly set to
                 * 'inline'. */
                $cd->setContentParamValue('attachment');
            }
        }
    }