Is there a QuickLook preview for plain-text files with unknown extensions?
You can use QuicklookStephen (https://github.com/whomwah/qlstephen).
Install via Brew:

brew cask install qlstephen


and then reset the Quicklook plugins:

qlmanage -r


and you should be good to go!

Tested with both files without extension and files with "unknown" extension (e.g. .nfo)