How can I find the backup for a specific file/mail, and view its contents?
Because CubeBackup keeps a version history of your Google data, including metadata for all files/emails (in a similar way to Git or Time Machine), and because these files are backed up incrementally and encrypted by default, it is usually not possible to view the contents of the backup files through the file system. However, you can always use the web dashboard to restore any data you like, and this does not overwrite the original files or backups.
To view the content of a file/mail, you can export emails/files from the backups. However, for security and privacy considerations, data export is disabled by default in CubeBackup. To enable the data export feature, please refer to this doc.