code: public void startPart(String contentType) throws IOException {
          if (inPart)
              out.write(__CRLF);
          inPart = true;
          out.write(__DASHDASH);
          out.write(boundaryBytes);
          out.write(__CRLF);
          if (contentType != null)
              out.write(("Content-Type: " + contentType).getBytes(StandardCharsets.ISO_8859_1));
          out.write(__CRLF);
          out.write(__CRLF);
      }
docs: Start creation of the next Content.
  
  @param contentType the content type of the part
  @throws IOException if unable to write the part