Support multiple image sizes for an attachment

[NfNitLoop]

Related to #20 -- maybe allow attaching multiple versions of an image with different sizes?

Hmm, this might be a bit complicated.

First thought: Add some "metadata" to the Attachment protobuf message that allows alternative sizes.

But ... wouldn't I want all attachments listed in the top-level repeated Attachments.file? Maybe there should be an Attachments.meta? But then clients that naively copy all Attachments may get redundant copies of images? Hmm. I'll need to think about this one a bit more.