Use the .appcache extension instead of .manifest, as per http://html5.org/r/5812 - thx mathias

mime.types

@@ -9,7 +9,7 @@ types {
     application/x-javascript              js;
     application/atom+xml                  atom;
 
-    text/cache-manifest                   manifest;
+    text/cache-manifest                   manifest appcache;
     text/mathml                           mml;
     text/plain                            txt;
     text/vnd.sun.j2me.app-descriptor      jad;

nginx.conf

@@ -90,7 +90,7 @@ http {
     expires 1M;
 
     # Static assets
-    location ~* ^.+\.(manifest)$ {
+    location ~* ^.+\.(manifest|appcache)$ {
       expires -1;
       root   /sites/example.com/public;
       access_log logs/static.log;