upload.dart

final client = Dio();
final url = 'https://some-api.dev/';

final directory = await getTemporaryDirectory();
final fileName = '${directory.path}/rant_about_the_world.txt';

final payload = FormData.fromMap({
  'user': 'marcossevilla',
  'essay': await MultipartFile.fromFile(fileName),
});

await client.post(
    '$url/user/new',
    data: payload,
    onSendProgress: (sent, total) {
      final progress = sent / total * 100;
      print(progress);
    },
);