@staticmethod
def denormalize_attachment(attachment):
    """convert attachment metadata from archivant to es format"""
    res = dict()
    ext = ['id', 'url']
    for k in ext:
        if k in attachment['metadata']:
            raise ValueError(
                "metadata section could not contain special key '{}'".format(k)
                )
        res[k] = attachment[k]
    res.update(attachment['metadata'])
    return res
