docs: def extensions(self):
          """
          Generate the regular expression to match all the known extensions.
  
          @return: the regular expression.
          @rtype: regular expression object
          """
          _tmp_extensions = self.mimes.encodings_map.keys() + \
              self.mimes.suffix_map.keys() + \
              self.mimes.types_map[1].keys() + \
              cfg['CFG_BIBDOCFILE_ADDITIONAL_KNOWN_FILE_EXTENSIONS']
          extensions = []
          for ext in _tmp_extensions:
              if ext.startswith('.'):
                  extensions.append(ext)
              else:
                  extensions.append('.' + ext)
          extensions.sort()
          extensions.reverse()
          extensions = set([ext.lower() for ext in extensions])
          extensions = '\\' + '$|\\'.join(extensions) + '$'
          extensions = extensions.replace('+', '\\+')
          return re.compile(extensions, re.I)