My best guess is that CloudFlare wrote this to prevent folks from serving big binary files like photo, music, or video and this txt file case was an unintended

My best guess is that CloudFlare wrote this to prevent folks from serving big binary files like photo, music, or video and this txt file case was an unintended condition that happens to work to CloudFlare's advantage. text/plain though is decidedly not text/html and I would expect CloudFlare to potentially do some on-the- fly optimizations that are aware of the structure of an html file that save terabytes a day at their scale.