Composing multipart/form-data with a different Content-Type on each parts with Javascript (or Angular)

> Wrong question asked, see my update below

I need to integrate my AngularJS Project with an existing RESTful API. These API consume POST request which upload a file, and also submit the form data in a request. Unfortunately, one of the form input requires to be in Content-Type: Application/json.

After search on the web, I could only POST with Content-Type: multipart/form-data in which each of the parts does not have a specific MIME. How can I compose my multipart/form-data with a different MIME for each parts in Javascript?

POST /api/v1/inventory
Host: localhost:8000
Origin: http://localhost:9000
Content-Type: multipart/form-data; boundary=------border

------border
Content-Disposition: form-data; name="owner"

john doe
------border
Content-Disposition: form-data; name="image"; filename="mybook.png"
Content-Type: image/png


------border
Content-Disposition: form-data; name="items"
Content-Type: application/json

{"name": "Book", "quantity": "12"}
------border--

Relevant References:

1. https://developer.mozilla.org/en-US/docs/Web/Guide/Using_FormData_Objects
2. https://stackoverflow.com/questions/9081079/rest-http-post-multipart-with-json
3. http://code.activestate.com/recipes/578846-composing-a-postable-http-request-with-multipartfo/
4. https://stackoverflow.com/questions/4007969/application-x-www-form-urlencoded-or-multipart-form-data
5. https://stackoverflow.com/a/9082243/764592

---

Update

Apologize for asking a wrong question. The original problem is that, I can see the server calling the logic something like,

func POST(req):
     owner = req.owner // This is string
     image = req.image // This is file object
     itemQuantity = req.items.quantity // Items is an object with attribute quantity
     itemName = req.items.name // Items is an object with attribute name
     

I have also managed to figure out how to submit such a post request. I will post my answer below.

Once again sorry for asking a wrong question.

According to the documentation of FormData, you can append a field with a specific content type by using the Blob constructor:

var formData = new FormData();

formData.append('items', new Blob([JSON.stringify({
    name: "Book",
    quantity: "12"
})], {
    type: "application/json"
}));

After careful observation, it turns out that it will send the part as follows:

Content-Disposition: form-data; name="items"; filename="blob"
Content-Type: text/json

The only alternative, safe from building the whole request yourself is to pass a string value:

formData.append('items', '{"name": "Book", "quantity": "12"}');

This, unfortunately, doesn't set the Content-Type header.

> Mistake #1: I mistakenly assume that the items has to be a json, so that we can call its attribute.

Solution: To submit a multipart request that contain a file and an object like format is very simple.

form = new FormData();
form.append('items[name]', 'Book');
form.append('items[quantity]', 12);
form.append('image', imageFile);
form.append('owner', 'John Doe');

So thus the request header and body will looks something like this

POST /api/v1/inventory
Host: localhost:8000
Origin: http://localhost:9000
Content-Type: multipart/form-data; boundary=------border

------border
Content-Disposition: form-data; name="owner"

john doe
------border
Content-Disposition: form-data; name="image"; filename="mybook.png"
Content-Type: image/png


------border
Content-Disposition: form-data; name="items[name]"

Book
------border
Content-Disposition: form-data; name="items[quantity]"

12
------border--

Nothing would get this to work, until I set the Content-Type header to undefined. In my case I am posting a file and some json.

public uploadFile(code: string, file):angular.IHttpPromise<any>{
    var data = `{"query":"mutation FIRMSCORECARD_CALCULATE($code:String!){ FirmScorecardMutation{ BatchCalculate(Code:$code) }}","variables":{"code":"${code}"},"operationName":"FIRMSCORECARD_CALCULATE"}`;
    var formData = new FormData();
    formData.append('operations', data);
    formData.append('file', file, file.name);
    
    let config = {
        headers: {
            'Accept': 'application/json',
            'Content-Type': undefined
        }
    };
    let response = this.$http.post(this.graphqlUrl, formData, config);
    return response;
}