How do I set a MIME type before sending a file in Node.js?

Search google for the Content-Type HTTP header.

Then figure out how to set it with http://expressjs.com/api.html#res.set

Oops, the example includes your answer ;)

Simply check the file ending, if it's .js, set the appropriate MIME type to make browsers happy.

EDIT: In case this is pure node, without express, look here: http://nodejs.org/api/http.html#http_response_setheader_name_value

I figured it out!

Thanks to @rdrey's link and this node module I managed to correctly set the MIME type of the response, like this:

function handler(req, res) {
    var url = convertURL(req.url);

    if (okURL(url)) {
        fs.readFile(url, function(err, data) {
            if (err) {
                res.writeHead(404);
                return res.end("File not found.");
            }

            res.setHeader("Content-Type", mime.lookup(url)); //Solution!
            res.writeHead(200);
            res.end(data);
        });
    } else {
        res.writeHead(403);
        return res.end("Forbidden.");
    }
}