I see that I can pass serializer as option but https://github.com/rdf-ext/rdf-normalize implements synchronous interface. Do I need to wrap rdf-normalize with serializer interface to use it with this store?

The default to only look for n-triples is a little cumbersome when those files are edited by hand. It would nice to have the option in the resolver to change the format. Could be as simple as adding the extension. It should be enough, given that rdf-utils-fs/fromFile already does recognise common file types.

const store = new FlatMultiFileStore({
  baseIRI: env.BASE_URI,
  path: '.store',
  extension: 'ttl' // where it would default to nt
})