config: add application/xml mimetype support.

application/xml is the older xml mimetype.  some legacy scripts make use of it,
as well as common use for debugging with curl and other tools.  so it makes sense
to have it alongside text/xml.

config/config.exs

@@ -26,6 +26,7 @@
   metadata: [:request_id]
 
 config :mime, :types, %{
+  "application/xml" => ["xml"],
   "application/xrd+xml" => ["xrd+xml"],
   "application/activity+json" => ["activity+json"],
   "application/ld+json" => ["activity+json"]