docs: def copy_contents(self, fileinstance, progress_callback=None,
                        chunk_size=None, **kwargs):
          """Copy this file instance into another file instance."""
          if not fileinstance.readable:
              raise ValueError('Source file instance is not readable.')
          if not self.size == 0:
              raise ValueError('File instance has data.')
  
          self.set_uri(
              *self.storage(**kwargs).copy(
                  fileinstance.storage(**kwargs),
                  chunk_size=chunk_size,
                  progress_callback=progress_callback))