code: def _build_chunk_headers(self):
          
          if hasattr(self, "_chunk_headers") and self._chunk_headers:
              return
  
          self._chunk_headers = {}
          for field in self._files:
              self._chunk_headers[field] = self._headers(field, True)
          for field in self._data:
              self._chunk_headers[field] = self._headers(field)
docs: Build headers for each field.