Currently HTTPSB supports only the https:// or https:// protocols for the matrix. file:// is ignored, and http resources pulled from a file://-based page are redirected through the chromium-behind-the-scene scope.

So investigate all the details in supporting the file:// protocol (from which no hostname can be extracted, thus no scoping, etc.). Main idea is to mash up all file://-based requests into a specific built-in scope, just like all orphan requests are mashed up into the chromium-behind-the-scene matrix.