Flutter url_launcher Attach file and email file

[flutter testing]

Requirement: I have to send file in the email. 

Problem: When user taps on the send icon, it calls launchURL method which opens up mail client without any issue. 

But if I add file to url variable, it does not attach the file.

launchURL method is given below. I will appreciate any help.

import 'package:url_launcher/url_launcher.dart';

  launchURL(toMailId, subject, body, file ) async {

    var url = 'mailto:$toMailId?subject=$subject&body=$body'; // This works fine

//    var url = 'mailto:$toMailId?subject=$subject&body=$body&file=$file'; // This does not attach the file

    if (await canLaunch(url)) {
      await launch(url);
    } else {
      throw 'Could not launch $url';
    }
  }