Using : ashow($headers);
With d=404 this is return :
Array
(
[cache-control] => max-age=300
[content-type] => text/problem-with-post-edit-buttontml; charset=utf-8
[date] => Sat, 16 Feb 2013 19:55:32 GMT
[expires] => Sat, 16 Feb 2013 20:00:32 GMT
[last-modified] => Mon, 11 Feb 2013 19:51:01 GMT
[server] => nginx
[via] => 1.1 varnish
[x-varnish] => 3531257088 3531246365
[content-length] => 13
[connection] => close
)<br /><br />Without d=404 this is return : <br />
Array
(
[access-control-allow-origin] => *
[cache-control] => max-age=300
[content-disposition] => inline; filename="ac38eb1eaad0c2c07ef90a0d2264f893.jpg"
[content-type] => image/jpeg
[date] => Sat, 16 Feb 2013 19:58:34 GMT
[expires] => Sat, 16 Feb 2013 20:03:34 GMT
[last-modified] => Mon, 11 Feb 2013 19:51:01 GMT
[server] => nginx
[source-age] => 1848
[via] => 1.1 varnish
[x-varnish] => 254322374 253216379
[content-length] => 3068
[connection] => close
)