Add Cache-Control header to media endpoints (#2612)

* Add Cache-Control header

* Raise rate_limiting threshold to 20

dendrite-sample.monolith.yaml

@@ -192,7 +192,7 @@ client_api:
   # and appservice users are exempt from rate limiting by default.
   rate_limiting:
     enabled: true
-    threshold: 5
+    threshold: 20
     cooloff_ms: 500
     exempt_user_ids:
     #  - "@user:domain.com"

dendrite-sample.polylith.yaml

@@ -195,7 +195,7 @@ client_api:
   # and appservice users are exempt from rate limiting by default.
   rate_limiting:
     enabled: true
-    threshold: 5
+    threshold: 20
     cooloff_ms: 500
     exempt_user_ids:
     #  - "@user:domain.com"

mediaapi/routing/routing.go

@@ -149,6 +149,9 @@ func makeDownloadAPI(
 			}
 		}
 
+		// Cache media for at least one day.
+		w.Header().Set("Cache-Control", "public,max-age=86400,s-maxage=86400")
+
 		Download(
 			w,
 			req,