code: public MultipartBuilder attachment(String content, String filename) {
          ByteArrayInputStream is = new ByteArrayInputStream(content.getBytes());
          return bodyPart(new StreamDataBodyPart(ATTACHMENT_NAME, is, filename));
      }
docs: Adds an attachment directly by content.
  
  @param content the content of the attachment
  @param filename the filename of the attachment
  @return this builder