Force Quick Look to treat .in files like .txt files I would like Quick Look to treat files that have .in extension like regular text files, meaning show me the contents of the file after I click spacebar on them. 

How can I configure my system to work that way?
Install QLStephen:

wget https://github.com/downloads/whomwah/qlstephen/QLStephen.qlgenerator.zip
unzip QLStephen.qlgenerator.zip
mkdir -p ~/Library/QuickLook
mv QLStephen.qlgenerator ~/Library/QuickLook
qlmanage -r