Obviously its configurable. The message on the config page said to let you know if there was a protocol that should be considered for inclusion by default.

This is configurable, and file:// is an unusual protocol to use as an editor.

Are you familiar with Linux? The protocol is not unusual, it's actually very common. It does not directly refer to an editor. Chrome passes the url to a program called "xdg-open" which proceeds to open it using your systems default application for that file type.

On OSX, this protocol is blocked by the browser in Safari, Chrome and Firefox in an almost-silent way. At least in Chrome, this is explicitly a security/policy issue. We have far more OSX users than desktop Linux users, and I don't want to include file:// in the default set because it looks easy but will fail non-obviously for most users.