Handle mime types

This extension should automatically handle the following mimes:
`text/markdown`
`text/x-markdown`
Currently in Firefox 102, it prompts the file picker on these mimes, while it should just open in the extension.
Yes I am aware that you can set your browser to handle markdown FTAs, however, this is not a proper solution.

No file or test is named. Start by locating the extension entry point that handles incoming MIME types, then reproduce the Firefox 102 behavior with text/markdown and text/x-markdown. Done means both MIME types open directly in the extension without showing the file picker.

Written by the indexing model from the issue text.