docs: def get_upload_content_type(self):
          """Determine the actual content type of the upload."""
          if not hasattr(self, '_upload_content_type'):
              with self.get_storage().open(self.get_upload_path()) as upload:
                  content_type = Magic(mime=True).from_buffer(upload.read(1024))
              self._upload_content_type = content_type
          return self._upload_content_type