Am un-aware of any plugin that provides a view of contents of a backup due to nature of how a .mbz is made. Example: if the course that was backed up had uploaded files as file resources in it, one would have to use the metadata in files.xml of the backup to find the contenthash value of the file (which looks like a bunch of numbers and letters) then use the first 4 characters to locate the file in the files directory of the backup (which looks like: /yy/zz/yyzzbunchoflettersnumbers.). For the humanly re-cognizable name of the file one would have to use the files.xml file.
If you have been saving your sites backups to a designated directory and not the sea of files in /moodledata/filedir/ then one could use command line and view contents via tar -tvf nameoffile.mbz
To see humanly recognizable info, one would have to pretend to restore to see the resources, then cancel the restore.
'SoS', Ken