docs: def multipart_init(self, bucket, key, size=None, part_size=None):
          """Initialize a multipart upload.
  
          :param bucket: The bucket (instance or id) to get the object from.
          :param key: The file key.
          :param size: The total size.
          :param part_size: The part size.
          :raises invenio_files_rest.errors.MissingQueryParameter: If size or
              part_size are not defined.
          :returns: A Flask response.
          """
          if size is None:
              raise MissingQueryParameter('size')
          if part_size is None:
              raise MissingQueryParameter('partSize')
          multipart = MultipartObject.create(bucket, key, size, part_size)
          db.session.commit()
          return self.make_response(
              data=multipart,
              context={
                  'class': MultipartObject,
                  'bucket': bucket,
              }
          )