Add object for model and core module

model/lib/extensions/session_extension.dart

@@ -0,0 +1,16 @@
+
+import 'package:jmap_dart_client/jmap/core/session/session.dart';
+import 'package:uri/uri.dart';
+
+extension SessionExtension on Session {
+
+  String getDownloadUrl(String accountId, String blobId, String name, String type) {
+    final downloadUriTemplate = UriTemplate('${downloadUrl.origin}');
+    return downloadUriTemplate.expand({
+      'accountId' : '$accountId',
+      'blobId' : '$blobId',
+      'name' : '$name',
+      'type' : '$type',
+    });
+  }
+}