Handle mime types

[brian6932]

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.

[Cimbali]

This is not supported by Firefox. Progress is tracked in this bugzilla bug. So I’m afraid the non-proper solution is the only one that has the merit of working, for now.

Note that our workaround on Linux (option 2) does not change file type associations however (i.e. which program opens which file type), but forces Firefox to consider markdown files as text/plain, and we then match all markdown file extensions. It only relies on modifying Firefox configuration, and nothing else.