Force re-cache after file has been modified

Under nginx obviously.

Let's say I have a npcdata.txt file that is getting ajax requested every time a new game is created on my online game. It's nice to cache that file using nginx, but if I make changes to npcdata, a user will not get that new information from that .txt file UNLESS they do a HARD reload on chrome.

Is there a way to have it so nginx automatically uncache the file if the file has been modified? And no, I do not want to add the ?=timestamp at the end of the url. That would defeat the whole purpose of having a cache in the first place :P

Thanks ~