expanded a comment around some of the new formpost callback usage

      return CURLE_HTTP_POST_ERROR;

    }

    /* set the read function to read from the generated form data */

    http->form.fread_func = conn->fread_func; /* get the previously set callback

                                                 function pointer */

    /* Get the currently set callback function pointer and store that in the

       form struct since we might want the actual user-provided callback later

       on. The conn->fread_func pointer itself will be changed for the

       multipart case to the function that returns a multipart formatted

       stream. */

    http->form.fread_func = conn->fread_func;

    /* Set the read function to read from the generated form data */

    conn->fread_func = (curl_read_callback)Curl_FormReader;

    conn->fread_in = &http->form;