can not specify the content type of MultiPartFile in Flutter

I figured out the answer and it is that I must import http parser package

you need to import:

 import 'package:http_parser/http_parser.dart';

Just check out this if it works

uploadFile() async {
    var postUri = Uri.parse("<APIUrl>");
    var request = new http.MultipartRequest("POST", postUri);
    request.fields['user'] = 'blah';
    request.files.add(new http.MultipartFile.fromBytes('file', await File.fromUri("<path/to/file").readAsBytes(), contentType: new MediaType('image', 'jpeg')))

    request.send().then((response) {
      if (response.statusCode == 200) print("Uploaded!");
    });
  }