Couldn’t find any documentation online or any updates about this except these tickets from 2 months ago.
Hey! Should totally work now (fixed over a month ago) 
sorry for the lack of update there, let me know if you’re seeing any issues
seems to have broken a few days ago: feat: implement as_file to inject a secret as a file instead of as a value by kfkonrad · Pull Request #250 · jdx/fnox · GitHub