I was just about to suggest this myself.
The ability to limit attachments of a post via tier would be INSANELY useful. It could be done via a simple dropdown list. And this would be independent of the post’s rating itself.
This would also be a great way to advertise a patreon’s content more.
Some examples: say you offer a download of your videos when you make them to patrons. You can make the video post public, but limit the video download to just the appropriate tier. Or if you’re an artist and you have source files as a reward option, you could include that with the original art post itself: normal tier patrons can see just the picture downloads, while the source file tier also can see the source file.
Another example, if you’re a NSFW artist, Patreon requires your posts to be limited to patrons only, to prevent non-patrons from downloading adult art. This would be a good workaround, where you could make censored or cropped images for the post itself, and make that public so non-patrons know what’s offered as a bit of a teaser, and then they can get the downloads to the uncensored images by pledging.
By default attachments could just use the tier of the post itself, so if you wanted the current functionality it’d still be preserved and would be just as fast to set up, and it would be backwards compatible with the current system.