upload method
Implementation
Future<PictrsUpload> upload({
required String filePath,
required String auth,
}) async {
final req = http.MultipartRequest('POST', Uri.https(host, extraPath))
..files.add(await http.MultipartFile.fromPath('images[]', filePath));
req.headers['Cookie'] = 'jwt=$auth';
final res = await req.send();
final Map<String, dynamic> body =
jsonDecode(utf8.decode(await res.stream.toBytes()));
body['instance_host'] = host;
return PictrsUpload.fromJson(body);
}