Adding support for MHTML files

[jan-tosovsky-cz]

Chromium-based browsers can save the complete webpage as a single Mime HTML file https://en.wikipedia.org/wiki/MHTML, but when opened locally, only HTML code is shown. Other resources like JS or CSS are blocked (Not allowed to load local resource).
Per https://stackoverflow.com/questions/39007243/cannot-open-local-file-chrome-not-allowed-to-load-local-resource answer I was hoping this Chrome app will help, but my MHTML file is shown as a plain text.