how to convert multipart/form-data to multipart/mixed

The document insert REST api (POST /v1/documents) requires multipart/mixed for content-type. All the examples online show how to use multipart/form-data. Based on my research I learned that multipart/mixed needs to be embedded in multipart/form-data. Can someone please point me to a example or resource where I can get a clue ? Thank you!

fyi: I'm using AngularJS on the front end and Node.js on the backend

Angular code:

         $http({
                url: '/new',
                method: 'POST',
                headers: {
                    'Content-Type': undefined
                },
                transformRequest: function(data, getHeaders) {
                    var form = new FormData();
                    form.append('bug', angular.toJson(bug));
                    for (var i = 0; i < data.files.length; i++) {
                        console.log('FORM',data.files[i]);
                        form.append('file' + i, data.files[i]);
                    }
                    return form;
                },
                data: {
                    bug: bug,
                    files: $scope.files
                }
            }).success(function(data, status, headers, config) {
                Flash.addAlert('success', 'successssss');
            }).error(function(data, status, headers, config) {
                Flash.addAlert('danger', 'failed');
            });

Node code:: using request NOTE: this code is certainly wrong since it taking data as json instead of mulitpart/mixed which I dont know and hence the question

....
....      
case 'POST':
            console.log('its a POST');
             var url = 'http://api-server.com:8003/v1/documents?extension=json';
            var options = {
                method: 'POST',
                headers: req.headers,
                url: url,
                body: JSON.parse(req.body.bug),
                json: true
            };

            req.pipe(request(options, function(error, response, body) {
                if (error) {
                    next(error);
                }
            })).pipe(res);

Here's the multipart/form-data that I get currently

------WebKitFormBoundaryJlYMd1KVllv1WgDS
Content-Disposition: form-data; name="bug"

{"relatedTo":[],"tickets":[],"id":1,"kind":"Other","createdAt":"2014-09-06T08:33:43.614Z","modifiedAt":"2014-09-06T08:33:43.614Z","status":"Verify","title":"Quae inventore beatae tempora mollit deserunt voluptatum odit adipisci consequat Est dolore quia perspiciatis","submittedBy":{"name":"Sudhakar Reddy","email":"sreddy@mycompany.com","username":"sreddy"},"assignTo":{"name":"Guzman Wagner","email":"guzmanwagner@mycompany.com","username":"small"},"description":"sdsdsdsds","category":"MLOS","tofixin":"Help-1.1","severity":"Performance","priority":{"level":"4","title":"Important"},"relation":"Test Specification task for","clones":[],"version":"6.0-3","platform":"EC2","memory":"Reprehenderit quia aut voluptatem in ex dolore eu numquam eum et esse officia id consequatur Est","processors":"Reiciendis nostrum adipisicing occaecat inventore veniam excepturi","note":"Officiis qui adipisci commodo eveniet, esse aperiam est non unde possimus, sed nesciunt, exercitation eius magna consequat. Sint ipsa, laboriosam.","changeHistory":[],"subscribers":[{"name":"Sudhakar Reddy","email":"sreddy@mycompany.com","username":"sreddy"},{"name":"Guzman Wagner","email":"guzmanwagner@mycompany.com","username":"small"}],"attachments":[{"webkitRelativePath":"","lastModifiedDate":"2014-07-18T23:53:29.000Z","name":"jamesbond.jpg","type":"image/jpeg","size":858159}]}
------WebKitFormBoundaryJlYMd1KVllv1WgDS
Content-Disposition: form-data; name="file0"; filename="jamesbond.jpg"
Content-Type: image/jpeg


------WebKitFormBoundaryJlYMd1KVllv1WgDS--

The difference between the different multipart/* content types is just semantics, so just modify the Content-Type in req.headers before sending the request to the REST endpoint:

var url = 'http://api-server.com:8003/v1/documents?extension=json';

// if you use `req.headers` elsewhere, you may want to make a copy of the
// headers object so as not to mutate the original headers ...
req.headers['content-type'] = req.headers['content-type']
                                 .replace('multipart/form-data',
                                          'multipart/mixed');

var options = {
    method: 'POST',
    headers: req.headers,
    url: url,
    body: JSON.parse(req.body.bug),
    json: true
};

req.pipe(request(options, function(error, response, body) {
    if (error) {
        next(error);
    }
})).pipe(res);