If you construct a filename, with a (future) type for it, then it might be valid to reject non-legal UTF-8 strings (and even QSN-encoding such, by default). But if you e.g. read files from your directly, we must support all files there, including non-UTF-8 (at least I feel like, by default, as I explained with my backup example).
I marked my comment with a solution, for QSN (seems least bad, no solution seems prefect), that I propose Julia would adopt, when you cast to a String.