We have a need for this at scrimba.com, where we are now developing a way for people to record coding casts with access to local files and your local terminal. We want to let people record from any directory using a simple CLI (you only need to install an npm package). This will start a websocket-server locally, and then open up your browser at scrimba.com - which connects to said socket. This works in chrome since we're allowed to connect to ws://127.0.0.1, but other browsers need another solution.
MITM is not a problem, since the only communication with the socket is to read files etc from your local machine, and a malicious third party have nothing to gain, and no real way to fool you.