I think the URL for the attchment should include the name of the file so as you can change the filename, in turn changing the URL and there is no worry about the cache (apart from if you are downloading a file which you have already downloaded previously). An example would be like with my gradient script, instead of
http://shoutbox.menthix.net/attachment.php?pid=695421 it would be
http://shoutbox.menthix.net/attachment.php?pid=69...radient_v3.00.plsc or something simular.