It mentions passing the header in the bud: https://github.com/hzrd149/blossom/blob/afa1290a6aa91fb5b371ae9d346d835cb0231f83/buds/06.md
Whoops, that's for using HEAD, but also applies to the PUT for /upload Let me know if you think you can pass this header in the request. Otherwise, I could see if I can infer it through extension.