You are always able to unlike then re-like a post, it's how I have been able to bypass the issue you have been having. It might take resetting the page after unliking the post but it should work 95% of the time.
that doesn't really work anymore, I would do that as well but now after you like a post when you come back the next time you're not able to unlike it, the like button isn't there anymore just the comment button. In order to come back and view the content again you can't leave the post liked.
Yeah, I can see how it would be annoying to not be able to unlike and relike a post (or just have the site remember it to begin with). Unfortunately the results are the same either way. If it's not hidden like this the files won't last and it's unreasonable to ask people to continually reupload stuff on a 90 page content thread.
Best advice I can give whether a link is hidden or not: Download anything you think you might want right away.