add method
Implementation
Future<String> add(MultipartFile f) async {
var request = MultipartRequest(
'POST',
Uri.parse(apiUrl + 'add'),
);
request.files.add(f);
final res = await request.send();
final r = await res.stream.bytesToString();
Map<String, dynamic> item = jsonDecode(r);
print(r);
return item['Hash'];
}