How to configure Jetty Server v12 to serve CSS and Javascript files

This is a followup question to my earlier question

The CSS file does not get loaded if I configure like this in the html files.

<link rel="stylesheet" href="./main.css" type="text/css">

or

<link rel="stylesheet" href="main.css" type="text/css">

While Edge and Chrome show no errors at all, Firefox shows this error message:

The stylesheet http://localhost:8080/app/main.css was not loaded because its MIME type, “text/html”, is not “text/css”.

In the Handler this is how the response header is set:

response.setStatus(200);
response.getHeaders().put(HttpHeader.CONTENT_TYPE, MimeTypes.Type.TEXT_HTML_UTF_8.toString());
Content.Sink.write(response, true, msg, callback);
callback.succeeded();
return true;

My question is: How to configure Jetty Server v12 to serve CSS and Javascript files?

Thanks in advance.