How to: Post a multipart image request with different content-types

I am trying to post to a Salesforce.com REST api that is expecting the following request -

POST /services/data/v33.0/chatter/feed-elements HTTP/1.1
Authorization: OAuth 00DRR0000000N0g!...
User-Agent: Jakarta Commons-HttpClient/3.0.1
Host: instance_name
Content-Length: 845
Content-Type: multipart/form-data; boundary=a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq
Accept: application/json
 
--a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq
Content-Disposition: form-data; name="json"
Content-Type: application/json; charset=UTF-8
 
{
   "body":{
      "messageSegments":[
         {
            "type":"Text",
            "text":"Please accept this receipt."
         }
      ]
   },
   "capabilities":{
      "content":{
         "description":"Receipt for expenses",
         "title":"receipt.pdf"
      }
   },
   "feedElementType":"FeedItem",
   "subjectId":"005RR000000DmOb"
}
 
--a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq
Content-Disposition: form-data; name="feedElementFileUpload"; filename="receipt.pdf"
Content-Type: application/octet-stream; charset=ISO-8859-1
 
...contents of receipt.pdf...
 
--a7V4kRcFA8E79pivMuV2tukQ85cmNKeoEgJgq--

I tried to write the following code but seems the two different content-type can not be recognised -

xhr.open("POST", postUri);
xhr.setRequestHeader("Authorization", authHeader);
xhr.setRequestHeader('enctype', 'multipart/form-data');
 
var data2send = {
    json: jsonText,
    feedElementFileUpload: imageBlob
};
xhr.send(postBody);

The major issue seems to be that the service can not recognise the first part (json) is application/json type. How can I indicate the different content-type for the two parts?