code: def encode_multipart_formdata(fields, boundary=None):
      
      body = BytesIO()
      if boundary is None:
          boundary = choose_boundary()
  
      for field in iter_field_objects(fields):
          body.write(b('--%s\r\n' % (boundary)))
  
          writer(body).write(field.render_headers())
          data = field.data
  
          if isinstance(data, int):
              data = str(data)  # Backwards compatibility
  
          if isinstance(data, six.text_type):
              writer(body).write(data)
          else:
              body.write(data)
  
          body.write(b'\r\n')
  
      body.write(b('--%s--\r\n' % (boundary)))
  
      content_type = str('multipart/form-data; boundary=%s' % boundary)
  
      return body.getvalue(), content_type
docs: Encode a dictionary of ``fields`` using the multipart/form-data MIME format.
  
      :param fields:
          Dictionary of fields or list of (key, :class:`~urllib3.fields.RequestField`).
  
      :param boundary:
          If not specified, then a random boundary will be generated using
          :func:`urllib3.filepost.choose_boundary`.