On upload, return URL from canonical server address and add extension

[laanwj]

Two changes to upload:

• Return the url relative to the canonical server name that was configured, instead of the one in the request. This makes it work better when running behind a reverse proxy.
• Add the extension based on the MIME subtype. For nostrudel (and possibly other nostr clients) this is required because otherwise it won't display the link as an image.

Edit: this needs some more thinking, i guess the first point should just work when using proxy_set_header Host $host;