This is OK for consistency! But I do not understand why it solves the problem.

res.set() is a thin wrapper around res.setHeader() with only minor conveniences, which should not matter because the value (2nd argument to res.set()) is already a string. Similarly, setting res.statusCode and using res.writeHead() are identical.

After further testing, I believe the problem happens because res.set is setting the charset parameter in the Content-Type header field, which is allowed. This is an issue therefore with mashlib which fails to render markdown, presumably because it is doing an exact string match to "text/markdown" and failing that rendering text.

I would revert all changes except L104 and L132, and look for why the render is misunderstanding the media-type.

You are right. Good catch
I checked in the browser network pane and effectively the issue is due to the charset parameter.

I will open an issue in mashlib
Adding back res.statusCode = resStatus to the proposed code should also work

Made some changes, this will work for now.