Mapping not being recognised in IIS 6 for HTTP Handler

Question

I've created a HTTP Handler to prevent users from accessing unauthorized streaming ".flv" files.

It implements the IHttpHandler interface, is referenced in the Web.Config and works when I map an extension to ".pdf", but when I add the ".flv" the mapping in IIS, the app still seems ignored it (even though everything else is the same as it was with the PDF extension)

I am running IIS on Windows XP.