That seems like the most likey lway that the backup could have been corrupted (given that it was uploaded and not in the same place where it was created). It seems that some browsers and/or OSes “helpfully” uncompress things immediately on download, or otherwise make it hard to tell that it’s been changed.
I think Windows makes it exceptionally hard to tell if that has happened, as it explicitly hides extensions by default.
If you’re using an OS where someone is likely to use those commands, it’s probably unlikely that it’s caused the problem. 