I think it may only work if you omit a Content-Type header. Checking Firefox's Network tab, it looks like the server isn't serving one for that page.
$ curl -I http://lcamtuf.coredump.cx/squirrel/
HTTP/1.1 200 OK
Date: Thu, 11 Aug 2016 05:18:00 GMT
Server: Apache
Last-Modified: Mon, 19 Sep 2011 23:31:49 GMT
Accept-Ranges: bytes
Content-Length: 135938
Content-Type: text/htmlI guess browsers only forbid ignoring Content-Type for stuff like JS, then. For JPEG it's probably not a security concern.