AFAIK none of the other fedi projects implement accesscontrol for files so far. If you know the URL you can access it without being authenticated.
This is an image shared privately with Mastodon:
https://files.mastodon.social/media_attachments/files/108/276/925/307/177/263/original/9bb0feff3a7bdfb0.pngThe same image shared privately with Hubzilla:
https://zotsite.net/photo/1b7d46ac-c9b1-4f59-9978-bcf35c9a64d1-0.png