Simple Nodejs web server always sends text/plain css and js files

I'm running into a weird problem when creating a simple web server in Nodejs. The http server runs fine and accepts requests and responses like it should. However, for some reason it always wants to send a content-type: of text/plain for everything. For example, .js and .css files always come down as text/plain whereas they should normally be sent as text/css or application/javascript. Chrome, the browser I'm using to test this, always complains about the MIME type of the resource:

Resource interpreted as Stylesheet but transferred with MIME type text/plain: "http://localhost:3000/test.css".

Resource interpreted as Script but transferred with MIME type text/plain: "http://localhost:3000/test-client.js".

What this ultimately means is the css is never applied to the page. I've added some logging and it appears that the http response is sending down the correct MIME type.

I've created a barebones version of what I'm doing. Hopefully someone can point out the flaw that I've coded:

test.js

var http = require('http'),
    fs = require('fs'),
    url = require('url'),
    path = require('path');
var contentTypes = {
    '.html': 'text/html',
    '.css': "text/css",
    '.js': 'application/javascript'
};
http.createServer(function(request, response) {
    // get file based on pathname
    var uri = url.parse(request.url).pathname,
        filename = path.join(__dirname, uri);
    fs.exists(filename, function(exists) {
        // if root directory, append test.html
        if (fs.statSync(filename).isDirectory()) {
            filename += 'test.html';
        }
        // figure out MIME type by file ext
        var contentType = contentTypes[path.extname(filename)];
        fs.readFile(filename, function(err, file) {
            // errors?
            if (err) {
                response.writeHead(404, {'Content-type:': 'text/plain'});
                response.write(err + "\n");
                response.end();
            } else {
                console.log('MIME TYPE for: ', filename , contentType);
                response.setHeader('Content-Type:', contentType);
                response.writeHead(200);
                response.write(file);
                response.end();
            }
        });
    });
}).listen(3000, function(){
    console.log("server started and listening on port 3000");
});

test.html

<!DOCTYPE html>
<html>
    <head>
        <link rel="stylesheet" href="test.css" type="text/css" />
    </head>
    <body>
        <h1>Test</h1>
        <div id="test"></div>
        <script type="text/javascript" src="test-client.js"></script>
    </body>
</html>

test.css

h1 {
    color: red;
}

test-client.js

var div = document.getElementById('test');
div.innerHTML = 'test client ran successfully';