code: private boolean isGroupMember(final String filter, final File candidate) {
      final String fileName = candidate.getName();
      final int lastDot = fileName.lastIndexOf('.');
      if (lastDot >= 0 && lastDot < fileName.length() - 1) {
        final String extension = ";" + fileName.substring(lastDot + 1).toLowerCase() + ";";
        final String semiFilter = ";" + filter + ";";
        return semiFilter.contains(extension);
      }
      return false;
    }
docs: Returns true if the file has an extension that
  appears in the group filter.
  
  @param filter
  String group filter
  @param candidate
  File file
  @return boolean true if member of group