Postgres is likely the best way to go and is what I have used before, GitHub - erleans/erleans: Erlang Orleans · GitHub
Mnesia can be fine on a single node but, particularly if running in kubernetes, don’t expect to be able to utilize it as a distributed store.
A StatefulSet which ensures the same volume mounted for a specific nodename could be useful or storing a local copy. But far better to be able to have a stateless app deployment and use Postgres for data.
This point really can’t be emphasized enough. If you’re going to use mnesia you should be aware of its use cases and its limitations. Mnesia might be fine for your use case but you should really prove that first.
I like to use jsonb columns in postgres for these sorts of things but since you specifically called that out as a non-option then I might consider something like dynamodb. Dynamo is a reasonable choice if you just want to store binary blobs somewhere. Its hard to tell from your example but if you only need to read in the files once on app start then you might be able to store the files on s3 and pull them from there when the app is booting. There’s a lot of options here. Most importantly I would highly recommend avoiding creating an ad-hoc database if you can help it.