I hadn't considered it! Your username is appropriate, too much to do :D

My requirements are pretty low so I was likely to just let it slide...

...but with your help (and perhaps from others on understanding the pieces), I'll consider it!

I'm remarkably unfamiliar with S3 style of services, only occasionally trying to dabble

edit: The choosing of appropriate headers may come down to Mattermost/clients -- BackBlaze might be technically doing the right thing saying "I don't know what to do with these"