Caching static files isn't working when using nginx as a reverse proxy

I have an ExpressJS app running behind nginx which I setup as reverse proxy. I've also setup some caching for static files like images, js and css. The problem is, they don't seem to actually be caching. The requests to those files always returns a 200 status code instead of a 304. If I inspect the headers in dev tools, the Expires header is never the same as it was the last time I requested the file. It always gets reset to a new value. Is there something I've missed in my configuration?I have an ExpressJS app running behind nginx wh