Nah, it checks to see if the value is a string. I used the convention in a couple places that when dumping to json, the actual payload goes in `_attachments` instead of couchdbkit's meta data. (See test: https://github.com/dimagi/dimagi-utils/pull/110/files#L1L43.)

In those cases, it signals that immediately after next save, those payloads should be added as attachments. Since they're also deleted from `_attachments`, you won't end up in a loop where all attachments get re-put on every save.