MartinJD flarum/markdown is one of the extensions that comes pre-installed with Flarum. It's just called "Markdown" in the admin panel, and enables formatting in posts with the markdown syntax. It can be used in addition or in place to bbcode.
This extension just requires that you keep it enabled. I suppose that's because it inserts the image using markdown syntax in the post and nothing would be rendered if Markdown isn't enabled.
