If anyone has continued to attach files over the past month, I'm afraid I have some not-so-good news. The first time that I used QBO following the introduction of this bug, I attached files to two transactions. After I found that those would not open in a separate browser tab but would instead bring up a Save dialog, I left those files attached but did not upload any more except to test (after which I would Cancel without saving the transaction).

This morning, with new attachments behaving normally, I went back and checked those earlier attachments. They still do not open in a new tab. So it looks like my intuition was correct on this: any attachments done during the bug period will have to be deleted and re-uploaded.