code: public void startPart(String contentType, String[] headers) 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);
          for (int i = 0; headers != null && i < headers.length; i++) {
              out.write(headers[i].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
  @param headers     the part headers
  @throws IOException if unable to write the part