We didn't Implement any privacy topics in regards of uploading media in private chat as it will end up in link to event. But form ux Perspective on uploading process, I will notify user about the media visibility to all, not just private chat members for now. I don't think Adding encryption is that straight forward as a group can be changed form private to public, and from close to open, then it add more complexity on visibility of media for other users.
It's good to think about the privacy in nip29 group, maybe introducing new events kind in nip29 to cover this aspects.