IHttpHandler ... Filter

Riiiiight!

I've writen an IHttpHandler in .Net to run as an ISAPI filter on all files on my server that have a .csv extension .. the reason for this is that they contain valuable data and I dont want any old shmo downloading them

So my filter basically picks up any hits to .csv files and performs a simple verfication using a parsed querystring guid and some encryption logic bla bla bla ... so basically it enforces you to have logged into the sites' security framework in order to access those files (As apposed to just typing the url to a .csv file and downloading it)

But now my problem comes in after the user have been validated and I now want to let him access the file .. how the hell do you force the server to action out his initial request ... all it does it bring up a blank screen (valid user) or redirects them to the login screen (invalid user) .. but if he's a valid user I want it to carry on the request and prompt the user with a download options box (the box that usally appears when you try and access a .csv file on the web) ..

SO! ... any ideas?

Shot