code: def create_screenshot(self, app_id, filename, position=1):
          
          # 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)
docs: 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