This applies to all file types. The images are cached by imagehoster so it remains available through the proxy. Notice this part of the URL that is the signature that expires, but Discord app recreates them:

ex=6864085a&is=6862b6da&hm=135739337563e755f7e01911125b69ae5e48298e8d0a43b2e48551b6e6df307f&