No, I really mean assets (uploads seem fine). It seems that when GlobalSettings.s3_cdn_url is defined, discourse expects assets (i.e., the javascript stuff) to be in the S3 CDN, so the site is broken because none of the assets are available.
It makes zero sense to me either. 
I just set SiteSettings.s3_cdn_url (not in GlobalSettings) and assets seem to still be coming from the server, not the CDN. So maybe it’s a bug with GlobalSettings.s3_cdn_url?
Yup. Re-setting the value in discourse.conf does this:
All those red things link to the CDN for the assets.
I turned it back on/off and defining it as a global makes assets link to the s3_cdn_url.
So I guess the thing to do is configure those values in a plugin and hide them from the UX that way.