Earlier quoted context omitted.
Perhaps you're describing a Browser Extension?
What would be the benefit of a native code extension over using a standalone application with some URL handler / file type integration? I don't see the advantage of "do it in the browser" here.
I don't see the advantage of "do it native" here. Nor do I see the advantage of doing in the browser. They both seem like perfectly good places to write a BitTorrent client. And I can certainly imagine some interesting cases where I would want to allow my site users to send traffic between each other over a well known, advanced P2P syndication system (rather than cobbling up my own).