Backup File format

I have upgraded multiple times and have never had a problem. Typically I have copied the entire SuiteCRM instance to a “Backup” folder and run the upgrade on it. If successfull I just move things over. If not I am no worse off. Well, this time I popped over the the “Backup” in the admin module and downloaded it to my system. Of course the upgrade tanked and my system is toast.

My question is what is the format of the backup? I have attempted to add a .zip suffix and it does not uncompress. What is the format and how do I get my files?