Bug attachments (served by file_download.php) have the usual PHP "please, do not cache this" headers (Expires, Cache-control) and no modification date, but each file is pretty much static data after its upload to the server.

This causes a performance impact because user agents are not allowed to cache these files, and every time you open a bug view page with images attached the browser downloads again the same static data.

I don't know if it is feasible to fix this, but I decided to report it to let you (developers) discuss about. The file_download.php script should also be able to answer HEAD requests to inform the user agent the last modification date of the file (that is, the upload time).