Changing a document's attachment from the web UI when the file is being edited from the client shouldn't be possible

Here is what I tried to do:

1. user1 opens a OOo document from the client, edits it and saves modifications. He keeps the document open.
2. From the web UI, user2 edits the document, changes the attachment for a .png file and saves.
3. user1 modifies the document in OOo and clicks on Save.
   A message says that the document has been modified since the last modification.
4. user1 clicks "save anyway" and closes the document.

In the end, what I get is:

• On the web UI, my document now has an image as main attachment.
• In user1 Drive folder, there is no image and the OOo file is still here with the latest modifications.