I was recently installing/uninstalling/re-installing some third-party Quick Look extensions (brew cask install qlstephen among others). When all the dust had settled everything worked well except Quick Look for .txt files no longer works (I get the default Dear Kate letter). Quick Look for .rtf files works fine.
By the way, the qlstephen extension is working OK for files without file extensions. I suspect that problem arose during the uninstalling/re-installing of everything. I'm running High Sierra.
Any suggestions for getting Quick Look working for plain text .txt files?