TF-629 Download file and get the contentType to open file in Android/iOS

lib/features/email/data/datasource_impl/email_datasource_impl.dart

@@ -1,3 +1,4 @@
+import 'package:core/core.dart';
 import 'package:dio/dio.dart';
 import 'package:jmap_dart_client/jmap/account_id.dart';
 import 'package:jmap_dart_client/jmap/mail/email/email.dart';
@@ -55,7 +56,7 @@ class EmailDataSourceImpl extends EmailDataSource {
   }
 
   @override
-  Future<String> exportAttachment(
+  Future<DownloadedResponse> exportAttachment(
       Attachment attachment,
       AccountId accountId,
       String baseDownloadUrl,