nginx appears to disregard gzip on a combined reverse proxy/sendfile server

Question

Could I ask once of you exports to look at this nginx reverse proxy and static asset server for a Rails 3.1 backend with precompiled assets? I would have expected all files to be served gzipped, but none of them are coming through that way.

nginx.conf:

worker_processes  1;                                                                                                                   

events {                                                                                                                               
        worker_connections  1024;                                                                                                      
}                                                                                                                                      

http {                                                                                                                                 
        include       mime.types;                                                                                                      

        keepalive_timeout  65;                                                                                                         

        include /usr/local/nginx/conf/sites-enabled/*;                                                                                 
}

sites-enabled/site.conf;

ssl_certificate /usr/local/nginx/conf/certs/site.com.crt;
ssl_certificate_key /usr/local/nginx/conf/certs/site.com.key;

server {
        listen 443 ssl;
        listen 80;
        server_name staging.site.com;

        gzip on;
        gzip_types  text/plain text/css application/x-javascript image/png image/jpeg; 

        location /assets {
                root /home/site/www/staging.site.com/current/public;
                expires 15d;
        }

        location / {
                proxy_pass http://127.0.0.1:1991;
                proxy_set_header Host $host;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_redirect off;
                proxy_set_header X-Forwarded-Proto https;
        }
}

Answer 1

I see a lack of gzip_proxied in your config, which won't be helping your case. I don't see a particular problem with it for static assets; my guess is that your MIME types aren't coming out right (a tcpflow of a request would be nice to inspect). A note, too: it's usually pointless to gzip PNGs/JPGs and other compressed image formats; there isn't enough spare entropy in them to make it worthwhile.