dabdab.org supports markdown with the help of goldmark. It's really hard to overestimate the usefulness of a decent parser. Since introducing image uploads I was really worried about hardcoding uploaded links into the message text. I assume message text almost immutable - it's weakly structured and any text updates will be quite painful.
What that means in practice - message text should contain as little representation details as possible and image urls is just one of such details. From now own any media upload will result in a simple <uuid>.png url, which will be properly resolved at the rendering time. The parser was really useful there since it's trivial to walk the parsed tree and update links if necessary