code: private String validExtension(String dataPath) throws TechnicalException {
          final Set<String> extensions = new HashSet<>();
          for (final File file : new File(dataPath).listFiles()) {
              if (FilenameUtils.getBaseName(file.getName()).equals(scenarioName) && FilenameUtils.getExtension(file.getName()).startsWith("xls")) {
                  extensions.add(FilenameUtils.getExtension(file.getName()));
              }
          }
          if (extensions.size() != 1) {
              throw new TechnicalException(Messages.getMessage(TechnicalException.TECHNICAL_EXPECTED_EXCEL_EXTENTION_ERROR));
          }
          return extensions.iterator().next();
      }
docs: Valid Excel extension: xls, xlsx, or xlsm.
  
  @param dataPath
  path of all files from input/output folder
  @return unique extension if right
  @throws TechnicalException
  If you are using Excel as a dataProvider, you must choose one of the following formats: xls, xlsx, or xlsm.