docs: private Workbook getWorkbook(final String fileName) throws IOException {
  		if (workbooks.get(fileName) == null) {
  			FileInputStream fiStream = new FileInputStream(new File(fileName));
  			fis.add(fiStream);
  			Workbook workbook = null;
  			try {
  				workbook = new HSSFWorkbook(fiStream);
  			} catch (OfficeXmlFileException e) {
  				fiStream.close();
  				fiStream = new FileInputStream(new File(fileName));
  				fis.add(fiStream);
  				workbook = new XSSFWorkbook(fiStream);
  			}
  			workbooks.put(fileName, workbook);
  
  		}
  
  		return workbooks.get(fileName);
  	}