protected function get_content_type($mail_type, $boundary)
	{
		$related = $this->config['force_mixed'] ? 'multipart/mixed; ' : 'multipart/related; ';

		switch ($mail_type)
		{
			case 'plain':
				return 'text/plain';
			case 'plain_attach':
			case 'html_attach':
				return $related.$boundary;
			case 'html':
				return 'text/html';
			case 'html_alt_attach':
			case 'html_alt_inline_attach':
				return 'multipart/mixed; '.$boundary;
			case 'html_alt_inline':
			case 'html_alt':
			case 'html_inline':
				return 'multipart/alternative; '.$boundary;
			default:
				throw new \FuelException('Invalid content-type'.$mail_type);
		}
	}