Upload a file with Rest API C# unity

Hi,

I need help on upload a file on Box.

 IEnumerator Upload()
    {
        byte[] imagebyte = File.ReadAllBytes("C:/Users/harshit.gaur/Desktop/Test11.txt");
        //Debug.Log("image " + imagebyte.ToString());
        attributes.name = "Test11.txt";
        attributes.parent.id = "166060886898";
        attributes.content_created_at = DateTime.Now.ToString();
        attributes.content_modified_at = DateTime.Now.ToString();
        string attri = JsonUtility.ToJson(attributes);
        List<IMultipartFormSection> form = new List<IMultipartFormSection>();
        form.Add(new MultipartFormDataSection("attributes", attri, Encoding.UTF8, "application/json"));
        form.Add(new MultipartFormFileSection("file", imagebyte, "Test11.txt", "application/octet-stream"));

        UnityWebRequest www = UnityWebRequest.Post(uploadUrl, form);

        www.SetRequestHeader("Content-Type", "multipart/form-data");

        if (playerToken != "")
        {
            print("URL " + playerToken);
            www.SetRequestHeader("Authorization", "Bearer " + playerToken);
        }
        yield return www.SendWebRequest();

        if (www.isNetworkError || www.isHttpError)
        {
            Debug.Log(www.downloadHandler.text);
        }
        else
        {
            Debug.Log("Form upload complete! " + www.downloadHandler.text);

        }
    }

With this code i am getting response {"code":"bad_request","help_url":"http://developers.box.com/docs/#errors","status":400,"message":"the request was rejected because no multipart boundary was found","type":"error"}

Can anyone help what is missing in this above ?