JavaScript MIME type is application/octet-stream nodejs

I'm writing a combiner module for node js. I'm using express to get the files like the following:

app.get('/combine/js/?files=scripts/file1.js;scripts/file2.js', function(req, res){
    res.contentType('text/javascript');
    res.end(content); //the combined files content
});

Now, when the page is loaded I'm getting the following error in chrome: 'Resource interpreted as Script but transferred with MIME type application/octet-stream'

What am I doing wrong?

UPDATE: this is the full app.get combiner function that response the content...

    app.get('/combiner/:type/?', function(req, res){
        var type = req.params.type;
        var files = [];
        files = req.query.files.split(';');
        var content = combiner.combine(type, files);
        switch(type){
                case 'js': res.contentType('text/javascript'); break;
            case 'css': res.contentType('text/css'); break;
        }
        content = content.replace('<:=appid=:>', vars.appid);
        res.end(content);
    });