We should provide better rules for Nginx.
The rules should redirect directly to existing webp files, and set Vary: Accept header when it does

Current rule is simply:

if ($http_accept ~* "webp"){
  rewrite ^/(.*).(jpe?g|png)$ /wp-content/plugins/webp-express/wod/webp-on-demand.php?wp-content=wp-content break;
}