use pcm as ext name when codec is pcm

Jellyfin.Api/Helpers/StreamingHelpers.cs

@@ -191,6 +191,11 @@ public static class StreamingHelpers
             state.OutputAudioBitrate = encodingHelper.GetAudioBitrateParam(streamingRequest.AudioBitRate, streamingRequest.AudioCodec, state.AudioStream, state.OutputAudioChannels) ?? 0;
         }
 
+        if (outputAudioCodec.StartsWith("pcm_", StringComparison.Ordinal))
+        {
+            containerInternal = ".pcm";
+        }
+
         state.OutputAudioCodec = outputAudioCodec;
         state.OutputContainer = (containerInternal ?? string.Empty).TrimStart('.');
         state.OutputAudioChannels = encodingHelper.GetNumAudioChannelsParam(state, state.AudioStream, state.OutputAudioCodec);