One of the settings for this service (category Additional settings) is Charset. This allows you to specify the file charset. Did you try setting this to UTF-8?

Unfortunately, long strings can't be 3GB, but rather almost 3.5MB. For any sane usage of JSON this should be enough, but I've seen REST APIs where e.g. PDF streams are returned in base64-encoded JSON properties, that come uncomfortably close to that size.

It would be really nice if the %DynamicObject class added support for streams to deal with these large payloads!