fix(stomp): pass the Content-Type from the MQTT message

apps/emqx_gateway_stomp/src/emqx_gateway_stomp.app.src

@@ -1,7 +1,7 @@
 %% -*- mode: erlang -*-
 {application, emqx_gateway_stomp, [
     {description, "Stomp Gateway"},
-    {vsn, "0.1.5"},
+    {vsn, "0.1.6"},
     {registered, []},
     {applications, [kernel, stdlib, emqx, emqx_gateway]},
     {env, []},

apps/emqx_gateway_stomp/src/emqx_stomp_channel.erl

@@ -1039,7 +1039,7 @@ handle_deliver(
                         {<<"subscription">>, Id},
                         {<<"message-id">>, next_msgid()},
                         {<<"destination">>, emqx_message:topic(NMessage)},
-                        {<<"content-type">>, <<"text/plain">>}
+                        {<<"content-type">>, content_type_from_mqtt_message(NMessage)}
                     ],
                     Headers1 =
                         case Ack of
@@ -1080,6 +1080,13 @@ handle_deliver(
     ),
     {ok, [{outgoing, lists:reverse(Frames0)}], Channel}.
 
+content_type_from_mqtt_message(Message) ->
+    Properties = emqx_message:get_header(properties, Message, #{}),
+    case maps:get('Content-Type', Properties, undefined) of
+        undefined -> <<"text/plain">>;
+        ContentType -> ContentType
+    end.
+
 %%--------------------------------------------------------------------
 %% Handle timeout
 %%--------------------------------------------------------------------