In this thread I have 2 similar attachments (Red Volkswagen T2 van):
I first uploaded it, but clicked cancel just when it reached 100%, it cancelled and was removed from the uploads section. Then I uploaded it again, but without cancelling.
However, when I posted the thread, the cancelled attachment also showed up.
This will probably be difficult to reproduce due to precise timing. But it should be impossible to cancel an attachment after it is saved to the database (or the request to do so is already send).