I'm in the same situation and don't have this issue. It's not that the OS doesn't support plain text files, it's that other OSs bundle in plain text editors, and iOS doesn't. Simply install one (like iAWriter), and you're done. You can't use iCloud for syncing, so then also install Syncthing (Mobius).

It's not as shiny a setup as the native offerings, but it's by no means a chore.