MSC3859: Add well known media domain proposal

proposals/3859-well-known-media-domain.md

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+# MSCXXX: .well-known Media Domain
+
+Currently Matrix clients and servers discover homeservers and identify servers via the `.well-known/matrix/client` and `.well-known/matrix/server` endpoints. This is currently limited to the homeserver and for clients the identity server. This MSC proposes adding an additional field that can be used to specify a media server that can optionally be used for `/_matrix/media/*` endpoints.
+
+The reasoning behind this change is to support hosting media behind a CDN designed and optimised specifically for serving media. Since media transfers and endpoints are very different to the other JSON based endpoints a separate domain offers maximum flexibility for server owners.
+
+
+## Proposals
+
+### Extend the client .well-known
+
+Add a new optional field, `m.media_server` that can specify a separate URL to be used for media requests. Clients can then optionally use this field for media endpoints:
+
+```
+{
+  "m.homeserver": {
+    "base_url": "https://matrix.example.com"
+  },
+  "m.media_server": {
+    "base_url": "https://matrix-media.example.com"
+  }
+}
+```
+
+To ensure backwards compatibility the homeserver base URL must also be able to serve the same media requests (via a proxy or some other backend system).
+
+### Extend the server .well-known
+
+This is the same as above but for the server endpoint:
+
+```
+{
+  "m.server": "matrix.example.com:443",
+  "m.media_server": "matrix-media.example.com:443"
+}
+```
+
+As above, the non-media endpoint must also serve media requests.