code: static OutputStream toXlsxFile(final IDataModel dataModel, final InputStream formatting) {
          ByteArrayOutputStream xlsx = new ByteArrayOutputStream();
          
          try { toWorkbook(dataModel, ConverterUtils.newWorkbook(formatting)).write(xlsx); }
          catch (IOException e) { throw new CalculationEngineException(e); }
          
          return xlsx;
      }
docs: Uses {@link DataModelConverters#toWorkbook(IDataModel, InputStream)} with {@link ByteArrayOutputStream} as out stream
  and new {@link XSSFWorkbook} as {@link InputStream} wrapper.