Consider orphaned attachment files

[joelvdavies]

Joshua pointed out an upload that fails after the original metadata is posted could be deleted by another user. This would leave the file in the object store while not being present in the database. Two thoughts come to mind:

1. We do the same as images and route the attachment upload through the API (this will increase the load on the service - particularly if there are many large uploads at once, but we could ensure deletion only occurs fully)
2. We have some sort of script to detect and remove orphaned attachments

It is possible even 1. cannot completely eliminate the possibility - if the first part of a multipart upload succeeds and the rest fails, although I suspect it would be a lot rarer.

[joelvdavies]

This will also now be possible for images as in #148 images are uploaded after the database insert, although again I think its highly unlikely to occur.