Client can and probably will use the "url" field returned from the /upload endpoint when composing kind 1 notes. although the GET /
If my main server cdn.hzrd149.com went down or the image was removed from it. clients could lookup my preferred list of blossom servers (kind 10063 event) and start going down the list asking each one for the exact same blob (without needing to know how the server works)
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Thats the point of requiring the "GET /