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Great question! We don't have a way to lock down Attachments natively, that's true even for read-only Observers on a board. Generally speaking, if someone can see content on a board, like an attachment uploaded to Trello's servers, they can open it.
Instead, I'd recommend using a third-party service like Google Drive to set permissions on who can access the content. For example, with Google Drive, permissions set on the file in Drive will pass down to Trello. Someone without permission to see the file in Drive will click on the Attachment link in the card and see an error.
Do let me know if you've more questions. I'd be happy to help.
Vishaal S | The Trello Team