how to send just a 200 response in express.js
Try this instead:
res.sendStatus(200);Or if you want to continue using explicitly defined headers, I believe res.end() needs to be called at some point. You can see how res.end() is utilized in the Formidable example.
The blank page is most likely the result of your client-side form handling. You may want to override the form's submit method and manually post to your express service to prevent the automated redirection you are seeing. Here are some other stackoverflow responses to a question involving form redirection. The answers are jQuery specific, but the basic idea will remain the same.