Ensure attachment data is replaced, not updated

When a new edition of a published document is created, it's attachments are duplicated and if changes are made to the new edition's attachments, those of the published edition should remain unchanged. This behaviour was broken during the recent refactoring of attachments, which meant that file changes to attachments on new draft editions are being seen instantly on the currently published ones. This commit reinstates the desired behaviour so that when a new file is uploaded on a draft edition, the other editions are not affected.