I'd like to avoid adding any "meta" endpoints to blossom servers, since that introduces and extra step for clients to intact with them and then the client needs to ask the server how to interact with it before it tries... things get messy

The /mirror endpoints works kind of similar though. the client makes a PUT /mirror request with the url and sha256 of the blob the user wants to mirror, then the server either mirrors it and returns 200 or refuses and returns 400 or 401