Ever tried to serve custom font files from a cdn or similar? Doesn't work too well in Firefox does it...
This middleware is designed to set the required headers for a cdn to cache, meaning you can leave your nginx conf alone.
Works well on heroku with cloudfront in front.
Add this to your applications Gemfile:
gem 'rack-access-control-headers'
And run:
$ bundle
In config/application.rb (or specific environment file):
config.middleware.insert_before 'ActionDispatch::Static', 'Rack::AccessControlHeaders', /assets/
Where /assets/ is a regex of the path you want to inject the headers.
N.b. including ahead of ActionDispatch::Static is important if you're running rails.