Add a screenshot to the web app identified by by ``app_id``.
        Screenshots are ordered by ``position``.

        :returns: HttpResponse:
            * status_code (int) 201 is successful
            * content (dict) containing screenshot data

def create_screenshot(self, app_id, filename, position=1):
        """Add a screenshot to the web app identified by by ``app_id``.
        Screenshots are ordered by ``position``.

        :returns: HttpResponse:
            * status_code (int) 201 is successful
            * content (dict) containing screenshot data
        """
        # prepare file for upload
        with open(filename, 'rb') as s_file:
            s_content = s_file.read()
        s_encoded = b64encode(s_content)
        url = self.url('create_screenshot') % app_id

        mtype, encoding = mimetypes.guess_type(filename)
        if mtype is None:
            mtype = 'image/jpeg'

        data = {'position': position,
                'file': {'type': mtype,
                         'data': s_encoded}}
        return self.conn.fetch('POST', url, data)