code: def get_ext(self, obj=None):
          
          if obj is None:
              obj = self._obj
          return self._extel.get_ext(obj)
docs: Return the file extension
  
          :param obj: the fileinfo with information. If None, this will use the stored object of JB_File
          :type obj: :class:`FileInfo`
          :returns: the file extension
          :rtype: str
          :raises: None