Fix UTF-8 encoding for trace file downloads

Set charset=utf-8 in Content-Type header to ensure proper display of Cyrillic and other Unicode characters in browsers.

Fixes #13952

(cherry picked from commit 8795c5d)

Author: bas0N

deps/rabbitmq_tracing/src/rabbit_tracing_wm_file.erl

@@ -34,7 +34,10 @@ serve(ReqData, Context) ->
     Content = rabbit_tracing_util:apply_on_node(ReqData, Context,
                                                 rabbit_tracing_wm_file,
                                                 serve, [Name]),
-    {Content, ReqData, Context}.
+    ReqWithCharset = cowboy_req:set_resp_header(<<"content-type">>,
+                                                 <<"text/plain; charset=utf-8">>,
+                                                 ReqData),
+    {Content, ReqWithCharset, Context}.
 
 serve(Name) ->
     Path = rabbit_tracing_files:full_path(Name),

deps/rabbitmq_tracing/test/rabbit_tracing_SUITE.erl

@@ -24,7 +24,8 @@ groups() ->
     [
       {non_parallel_tests, [], [
                                 tracing_test,
-                                tracing_validation_test
+                                tracing_validation_test,
+                                trace_file_content_type_test
                                ]}
     ].
 
@@ -128,6 +129,35 @@ tracing_validation_test(Config) ->
     http_delete(Config, Path, ?NO_CONTENT),
     ok.
 
+trace_file_content_type_test(Config) ->
+    case filelib:is_dir(?LOG_DIR) of
+        true -> {ok, Files} = file:list_dir(?LOG_DIR),
+                [ok = file:delete(?LOG_DIR ++ F) || F <- Files];
+        _    -> ok
+    end,
+
+    Args = #{format  => <<"text">>,
+             pattern => <<"#">>},
+    http_put(Config, "/traces/%2f/test-charset", Args, ?CREATED),
+
+    Ch = rabbit_ct_client_helpers:open_channel(Config),
+    amqp_channel:cast(Ch, #'basic.publish'{ exchange    = <<"amq.topic">>,
+                                            routing_key = <<"key">> },
+                      #amqp_msg{props   = #'P_basic'{},
+                                payload = <<"Test message">>}),
+    rabbit_ct_client_helpers:close_channel(Ch),
+
+    timer:sleep(100),
+
+    http_delete(Config, "/traces/%2f/test-charset", ?NO_CONTENT),
+    {ok, {{_HTTP, 200, _}, Headers, _Body}} =
+        req(Config, get, "/trace-files/test-charset.log", [auth_header("guest", "guest")]),
+    ContentType = proplists:get_value("content-type", Headers),
+    ?assertEqual(match, re:run(ContentType, "text/plain", [{capture, none}])),
+    ?assertEqual(match, re:run(ContentType, "charset=utf-8", [{capture, none}])),
+    http_delete(Config, "/trace-files/test-charset.log", ?NO_CONTENT),
+    ok.
+
 %%---------------------------------------------------------------------------
 %% TODO: Below is copied from rabbit_mgmt_test_http,
 %%       should be moved to use rabbit_mgmt_test_util once rabbitmq_management