mbz file is not regognised by Moodle

Using Moodle 2.87

I have made a backup of a course (.mbz), downloaded, then changed extension  to .zip and unzipped.

Made some translations and zipped again, then extensions changed to .mbz.

The problem is that when uploaded for restoring, a message appears that it is not a Moodle file

What I'm doing wrong?  Where could be the problem?

Thanks in advance