Fixed attachments (that is, pictures)

Fixed four separate bugs, one obvious, two sad and one bizarre.

The straightforward one is that the main attachment entry point hadn't
caught up to the big change from Spaces being User-owned to
Identity-owned.

The sad ones were simply that the error logic in SpaceManager was
horrible, and that we were returning an error type that no longer made
sense in the current message structure.

The bizarre one was that, for some reason, blobs are suddenly returning
as Some(Array[Byte]), whereas they had previously been returning
Array[Byte].  That's really disturbing -- I don't understand what
changed...