There's a test for that case, and it looks like `IMFMediaSink_AddStreamSink` may also return `E_INVALIDARG` in that case. It's flagged as broken
I can't find that tests, can you tell me where it is? I do find a IMFMediaSink_AddStreamSink test will NULL type which is flagged as broken, but it's for VideoRenderer in test_evr(), not for media sink(file sink).