docs: private Multipart buildContent(Charset charset) throws MessagingException {
  		final Multipart mainPart = new MimeMultipart();
  
  		// 正文
  		final BodyPart body = new MimeBodyPart();
  		body.setContent(content, StrUtil.format("text/{}; charset={}", isHtml ? "html" : "plain", charset));
  		mainPart.addBodyPart(body);
  
  		// 附件
  		if (ArrayUtil.isNotEmpty(this.attachments)) {
  			BodyPart bodyPart;
  			for (DataSource attachment : attachments) {
  				bodyPart = new MimeBodyPart();
  				bodyPart.setDataHandler(new DataHandler(attachment));
  				bodyPart.setFileName(InternalMailUtil.encodeText(attachment.getName(), charset));
  				mainPart.addBodyPart(bodyPart);
  			}
  		}
  
  		return mainPart;
  	}