When comparing a draft with its published counterpart, attachment list changes are not correctly computed

Steps to reproduce

• Create draft with some content
• Publish it
• Add attachment to draft document
• Go to the published document, hit "Unpublish"

Expected result

Before proceeding with the unpublication, a message warns that the draft differs from the published document and offers the option to overwrite the draft.

Actual result

The displayed message shows that the draft and the published documents are identical.