Client can and probably will use the "url" field returned from the /upload endpoint when composing kind 1 notes. although the GET / serves a different purpose
Take this note for example nostr:nevent1qvzqqqqqqypzqfngzhsvjggdlgeycm96x4emzjlwf8dyyzdfg4hefp89zpkdgz99qyghwumn8ghj7mn0wd68ytnhd9hx2tcpzfmhxue69uhkummnw3e82efwvdhk6tcprfmhxue69uhhq7tjv9kkjepwve5kzar2v9nzucm0d5hsqgp40020uraxgjhz7qlaspnvwfu782qgulw7vrqqe2q45su6kqywkss9rclp
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 /" endpoint to be in the exact same location on every server. so its super easy for clients to quickly fallback and check other servers
