Cannot submit form with supertest and mocha - mocha.js

Question

I am using supertest and mocha to test a nodejs application. One of the things users can do is to submit a very simple form, which is picked up by the node server and parsed using formidable.
Here is the mocha test code:
var should = require('should'),
express = require('express'),
app = require('../app.js'),
request = require('supertest'),
csrfToken,
sessionId,
cookies = [];
describe('Post Handler', function(){
it('Uploads new post', function(done){
var req = request(app).post('/post?_csrf=' + csrfToken);
req.cookies = cookies;
req
.type('form')
.send({fieldTitle: 'autopost'})
.send({fieldContent: 'autocontent'})
.send({contentType: 'image/png'})
.send({blobId: 'icon_23943.png'})
.expect(200)
.end(function(error, res){
console.log('here');
done();
});
});
csrfToken retrieves a csrf token from the server, since I am using the csurf module and every POST method requires a csrf token. cookies stores the session cookie that is provided by the node server so I can persist the session between requests.
The form is processed by the following code:
//Takes HTTP form posted by client and creates a new post in the Db
exports.postPostUpload = function (req, res) {
var form = new formidable.IncomingForm();
form.parse(req, function (err, fields, files) {
console.log(err);
if (err) res.redirect(303, '/error');
else {
var new_post = new post_model.Post().createNewPost(fields);
new_post.setUserId(req.session.passport.user.userId);
new_post.uploadPostToDb(function (error, result) {
if (error) return res.status(500).end();
else {
if (new_post.media.contentType.indexOf('video') !== -1) {
addMessageToEncodingQueue(new_post, function (error, result, response) {
if (error) {
errorHelper.reportError({
stack: new Error().stack,
error: error
});
res.status(500).end();
}
else res.status(200).send(new_post.cuid);
});
}
else return res.status(200).send(new_post.cuid);
}
});
}
});
}
My current problem is, that once the form handler executes the line form.parse(req, function (err, fields, files) {, nothing happens. Formidable does not return error, it just does not return anything. Consequently, the mocha test never receives a reply from the server, and eventually the socket hangs and the test crashes. Needless to say, the form is successfully submit if you do it manually via the website.
There must be an error in the way supertest/mocha are executing this test, but I have not been able to find it. Any pointers are highly appreciated.