Matt Jones

We could pull in a `multipart` dependency with a `file_upload` feature flag so that users can opt into support or not.

Updating this PR to also include adding support for `users.setPhoto`, as it is unsupported for the same reason.

> maybe the EventHandler could have an on_error() fn or something? I don't entirely disagree with this idea (we discussed it quite a bit in a past PR), but perhaps...

Hrm, yeah... attachments are a bit special, since they're the only type that actually gets passed as a request field so they need serialization. There's no standard Attachment type definition...