If-Modified-Since should be honored when downloading attachments

As per http-1.1 (not yet an official thing I fear), and as done by all browsers around the earth, a "fearful caching" is the most widespread practice: for most "attached objects", a request is made with all the normal data with the addition of an extra header "If-Modified-Since" which indicates the modification date recorded for the entity that the client has in cache.

A normal server (e.g. Apache HTTPD, Tomcat's file-servlet, Jetty's file servlet) then responds 304 Not Modified as status, without any further details, indicating that this cache at the client side is good.

Lack thereof is very visible at, for example, http://i2geo.net/xwiki/bin/view/Coll_Pech/Parabola_0 where the profile picture is a huge file (for slow network, 300 kb) and is reloaded everytime you come back to page. The platform above is running Curriki 1.8 branch, itself running xwiki 1.5.4-curriki4.

I believe this still holds for modern xwikis as well.