code: def per_file_type_data(self):
          
          ret = {}
          for cache_date in self.cache_dates:
              data = self._cache_get(cache_date)
              if len(data['by_file_type']) == 0:
                  data['by_file_type'] = {'other': 0}
              ret[cache_date] = data['by_file_type']
          return ret
docs: Return download data by file type.
  
          :return: dict of cache data; keys are datetime objects, values are
            dict of file type (str) to count (int)
          :rtype: dict