OK, that makes more sense, thanks.
I do think that while it was due to carelessness, you have some culpability. You did not do the assignment, yet submitted something.
You may well have to bear some consequences for that beyond getting a zero on the assignment.
It doesn't sound like expulsion is likely or deserved, though.
I think you just have to present it exactly as you say here (your latest post, your first one was a bit confusing), with a copy of the document in question and a copy of the assignment itself.