The problem has nothing to do with extensions per se. The server has to have the MIME-type (also seen in the HTTP header as "Content-type:") defined as being associated with that extension, so that it adds the right line to the header when it sends it back out. I've successfully served MTS files from an IBM OS/390 CTS system that doesn't even know what a file extension is, because the server code gets the Content-type right. Your problem is that the sysprogs who run the server are not doing their job.