Don't use s3/CDN url for media attachments
When you use s3, media links in new posts are made directly. For example, the media URL of this post: https://pl.busshi.moe/notice/9vWIy7854XDg7edBYm is https://blobcdn.busshi.moe/plbusshimoe/4ba00ed726a11321f3936ff780a19c32b1930f4ae77d83ce0e4f2421cf793086.png
This means that if I wanted to migrate away from s3, or change my bucket URL, media URLs for all old posts would be broken.
Therefore, the media URL that should be used should always be https://pl.busshi.moe/media/4ba00ed726a11321f3936ff780a19c32b1930f4ae77d83ce0e4f2421cf793086.png (example), since that always redirects to where the media is currently hosted.