Clojurians Log v2

You can see it is the markup plugin that actually reads the file and at once turns it into HTML https://github.com/cryogen-project/cryogen-flexmark/blob/master/src/cryogen_flexmark/core.clj So you could e.g. make your own impl. of the Markup protocol that copies what cryogen-flexmark does but also adds the changes you need.

Sorry, I don't understand

I need post.content as a raw markdown instead of HTML DOM

eventually something to HTML -> text

I guess you are on your own then. Look at the code, it is quite straightforward.