Beginning vr 2.6 there was an experimental option to use tar.gz for compression (backups). Had to be turned on/chosen. Option still existed in 2.7/2.8 and at 2.9 it became the default.
So, the .mbz file created may have been a tar.gz file, not a zip.
First, you don't really have to change the extension to .zip ... that's only for operating systems that need 3 letter extensions.
You've not mentioned what OS you have ... server or desktop/workstation, so in general install something that can create/extract tar.gz's. Mac's have that built in. Linux boxen should also. Windows requires installation of software.
If Mac or Linux, open a terminal session in the folder/directory where a downloaded .mbz file exist. The issue:
file -b [nameofbackupfile].mbz
If the .mbz file is a zip you will see:
Zip archive data, at least v2.0 to extract
If the .mbz file is a tar.gz you will see:
gzip compressed data, from Unix
How to from command line:
Download the .mbz file to a test directory on local machine (workstation).
Mac/Linux: open terminal and change into the 'test' directory.
From the test directory location issue:
tar zxvf [nameofbackupfilename].mbz [ENTER]
switches in above command: z (using gzip), x extract, v verbose (show what doing) and f for tar working with a file.
IF the .mbz file is valid, the command above should un-archive without error. Valid backup file confiirmed.
To use .mbz to restore turn on experimental option if OFF (2.7/2.8 - remember, it's on by default in 2.9). Then restore.
'spirit of sharing', Ken