docs: def post(self, bucket=None, key=None, uploads=missing, upload_id=None):
          """Upload a new object or start/complete a multipart upload.
  
          :param bucket: The bucket (instance or id) to get the object from.
              (Default: ``None``)
          :param key: The file key. (Default: ``None``)
          :param upload_id: The upload ID. (Default: ``None``)
          :returns: A Flask response.
          """
          if uploads is not missing:
              return self.multipart_init(bucket, key)
          elif upload_id is not None:
              return self.multipart_complete(bucket, key, upload_id)
          abort(403)