by Bob Belz.
Thanks, it is tgz in 3.5.2. On mac (or Linux) command line this unzipped into a folder
$ cp backup-course-20190417-1105-nu.mbz backup-course-20190417-1105-nu.tgz
$ mkdir course_backup
$ tar -xzvf backup-course-20190417-1105-nu.tgz -C course_backup