I explained a bit in the commit message - because mass-assignment of nested attributes protects assignment of the `type` column, we have lots of `Attachment`s in the database created using the old interface that's used on corporate information pages and policy advisory groups.

It's one of the reasons why we wanted the UI for managing attachments on these things to be updated not to use nested-attributes. Now that @evilstreak has done that work, we can tidy up the DB, make `Attachment` abstract and move the file-related logic out of the base `Attachment` class and into `FileAttachment`. But reckon that needs to be another story.