Flutter form with attachment (file upload) - flutter

Question

I am trying to build a Flutter form with 2 text fields and 1 field for an attachment:
Text Field 1
Text Field 2
Attachment
Send Data Button
This is to make a POST request and send these 3 values to my server, store the file locally and get the name of the file in the database. This backend logic is already covered.
I was trying to use a MultiPart request as follows:
Future makePostRequest(
String title, int subjectid, int teacherid, String limitDate) async {
var url = Uri.parse('http://localhost:8000/newHomework');
var request = http.MultipartRequest('POST', url);
var headers = {'Content-Type': 'text/plain; charset=UTF-8'};
print(this.file);
Uint8List data = await file.readAsBytes();
List<int> list = data.cast();
request.files
.add(http.MultipartFile.fromBytes('text', list, filename: 'tarea1'));
request.headers.addAll(headers);
request.fields['title'] = title;
request.fields['limitDate'] = limitDate;
request.fields['subjectid'] = subjectid.toString();
request.fields['teacherid'] = teacherid.toString();
request.fields['ext'] = '.txt';
var res = await request.send();
return res.stream.bytesToString().asStream().listen((event) {
var parsedJson = json.decode(event);
print(parsedJson);
});
}
But I do not know how to do this:
Include a button in the form to add the attachment (will only be text files, and only 1 file always) and then send all of this information with the Send Data Button.
Maybe I am approaching this from an incorrect perspective... Any ideas on how to do this?
Update makePostRequest method, now getting error: Error: NoSuchMethodError: 'readAsBytes'