This should now be fixed.
Ok. Here are the request/response headers from your attachment:
Request URL:[URL]https://z22se.co.uk/attachments/2012-09-19-09-50-24-jpg.8944/[/URL]
Request Method:GET
Status Code:200 OK
Request Headers
Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Cache-Control:max-age=0
Connection:keep-alive
Cookie:<removed>
Host:[URL='http://www.z22se.co.uk']www.z22se.co.uk[/URL]
Referer:[URL]https://z22se.co.uk/threads/engine-will-not-start-after-rebuild.21863/[/URL]
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/537.17 (KHTML, like Gecko) Chrome/24.0.1312.57 Safari/537.17
Response Headers
Cache-control:private, no-cache, no-store, proxy-revalidate, no-transform
Connection:Keep-Alive
Content-Disposition:inline; filename="2012-09-19 09.50.24.jpg"
Content-Encoding:gzip
Content-Type:image/jpeg
Date:Sun, 03 Feb 2013 23:54:12 GMT
Etag:1352049415
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Keep-Alive:timeout=5, max=100
Last-Modified:Sun, 03 Feb 2013 23:54:12 GMT
Pragma:no-cache
Server:Apache
Transfer-Encoding:chunked
Vary:Accept-Encoding,User-Agent
X-Content-Type-Options:nosniff
X-Powered-By:pHP/5.4.11
Here are the headers for an attachment on my test forum:
Request URL:http://<mytestsite>/attachments/dunmorogh_ridgecamp-png.28/
Request Method:GET
Status Code:304 Not Modified
Request Headers
Accept:text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-US,en;q=0.8
Authorization:<removed>
Cache-Control:max-age=0
Connection:keep-alive
Cookie:<removed>
Host:<mytestsite>
If-Modified-Since:Sun, 03 Feb 2013 01:26:51 GMT
If-None-Match:1359247650
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/537.17 (KHTML, like Gecko) Chrome/24.0.1312.57 Safari/537.17
Response Headers
Cache-control:private, max-age=0
Connection:Keep-Alive
Date:Sun, 03 Feb 2013 23:58:12 GMT
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Keep-Alive:timeout=5, max=150
Server:Apache
You can see that "If-None-Match" is not being submitted when I request your attachment, but it is being submitted for my attachment. That is the nature of the problem. The question becomes why is it missing from the request? I believe it is related to these response headers from your forum:
Cache-control:private, no-cache, no-store, proxy-revalidate, no-transform
Pragma:no-cache
That would instruct my browser to not cache the resource.
I can't find where this response header is coming from. Perhaps something from your Apache config?
So, the issue was with a setting we had in the .htaccess file that was "supposed" to improve caching, but was causing the problem on here with lightbox.
# Force no caching for dynamic files
<FilesMatch "\.(php|cgi|pl|htm)$">
ExpiresActive Off
Header set Cache-Control "private, no-cache, no-store, proxy-revalidate, no-transform"
Header set Pragma "no-cache"
</FilesMatch>
I've removed all the bits I added in (was done quite a while back now), and it's working as expected once the initial download of all the images has completed.
