feat: Add text/markdown as supported mime type

[vmiklos]

This is related to
CollaboraOnline/online#16018.

Summary

Once both https://gerrit.collaboraoffice.com/c/online/+/7180 is submitted and this PR is merged, users can edit markdown files in COOL, which was inconsistent, since convert-to worked, but not interactive editing.

[vmiklos]

Hi @elzody @juliusknorr , could you please review this when you have time? Thanks.

We recently noticed that while the COOL convert-to API supported markdown and the postmessage API also supports markdown copy&paste, actual open, edit, save of .md files is not working with COOL. This is to fix it. Use-case is something like opening a .md snippet in COOL, then you give some polished look like adding an image in the header, a page number in the footer interactively, then export to PDF or something like that.

[elzody]

I had the same thought as Caolán in this comment. If we add text/markdown support it might ideally go into the MIMETYPES_OPTIONAL array - that way we can keep the Text app as the default but still allow the user to edit Markdown files with Collabora if they wish. We do this already with PDFs (see src/file-actions.js).