"Missing Attachments" UI #80

Sometimes attachments fail to get uploaded. (Ex: from feotweet.) It's a bit of a PITA to manually curl the attachment to the correct location. Would be handy if there was a way to just drag an attachment to an existing post and have the web client PUT it for us.

We even know what its size and hashes should be, so we could just let a user drop a file, check its size and hash, then upload it to the correct location if it matches the criteria.

For now, I'm thinking of this on a single post basis, but eventually we could maybe expand the functionality to look for missing attachments across a span of posts.