Describe your feature request
Additional context
Can you do QuickLook on it?
QuickLook is not enough.
To modify (annotate) PDFs, I have to open with external editor (PDF Expert).
Also, there are many files does not have QuickLook extension.
BTW, QuickLook seems to be supported only for TextBundle.
I understand the need to open/view attachments.
But I'm not sure what you mean regarding QuickLook on TextBundle. Is this in Finder or FSNotes? Does QuickLook window have an Open/Edit button or not?
These questions are to gauge how much work is needed to implement the feature.
TextBundle's context menu:

Pure markdown's context menu:

If you choose "Quick Look Attachments", the top of the QuickLook window looks like this:

So you can choose to "Open with Preview" (default app for that file type) there.
Not ideal, but a temporary workaround.
Nice catch, will be implemented in next release 馃憣 I think attachments should be more polished.