To server static files from nginx use the below configs inside serverblock of nginx,

location ~* \.(js|css|png|jpg|jpeg|gif|ico|eot|otf|ttf|woff)$ {
access_log off; log_not_found off; expires 30d;
}

and use symbol "|" without quote and extension name to add more static file or extension types.

Source: http://atulhost.com/install-wordpress-on-nginx-with-fastcgi-cache-in-ubuntu