... or the downstream program could ask the upstream one (or get automatically along with the stream) about meta-data/type information for the stream it is bein

... or the downstream program could ask the upstream one (or get automatically along with the stream) about meta-data/type information for the stream it is being passed, and then it could benefit fully from already-known information. Though that does not solve the need to potentially read the full stream and buffer it before doing the processing.