Ensure all attachment records have a type set

Currently a number of attachments in the database have a blank STI
"type" attribute. This is because we were previously creating
attachments using a nested attributes-based UI, which doesn't allow
the type to be set. We no longer use nested attributes for managing
attachments so we are free to fix this data now.
Merge pull request #1243 from alphagov/fix-attachment-type-data

Ensure all attachment records have a type set