while technically you could still set the database as your attachment storage (there is only no way to go from files back to database, but you could start fresh with attachments in the database), I'd really not recommend to do so, since attachments usually account for 75-90% of your total storage, and no one wants to handle databases of multiple terabytes in size.
Like already written in https://forum.kopano.io/topic/370/kopano-core-high-availebility i would also urge you to reconsider Galera. While a master-master replication is a valid idea, Galera to my knowledge has the downside of storing a full replica of all your databases on each node, again not really a good idea when thinking of multiple terabytes of emails and attachments. Not to mention other potential applications stealing i/o from your kopano database.
But if you really want to go for attachments in db, you need to modify
ucr set kopano/cfg/server/attachment_storage=database (typed from the top of my head so the actual syntax may differ) and stop your server before and drop your old database.
Other options are of course to make the drive you store your attachments on "high available" (latency is not really an issue for us, since attachments are mostly on access) or use our s3 backend and have a ha s3 storage.