To improve security of Discord's CDN, attachment CDN URLs have 3 new URL parameters: ex, is, and hm. Once authentication enforcement begins later this year, links with a given signature (hm) will remain valid until the expiration timestamp (ex). To access the attachment CDN link after the link expires, your app will need to fetch a new CDN URL.

“Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive. It would be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience.” — C.S. Lewis