Even if windows did some upmixing (which I find hard to believe... ) I don't think it could cheat the reciever ...
Maybe there's some setting somewhere that needs to be adjusted but I really do not know where to look ...
Re: upmixing - it is possible, but of course if you use some player. Some players, iirc, can even do backtranscoding PCM->AC3 for receiver. When playing in Windows, do you have the "Dolby" indicator turned on?
There is possibility that ffmpeg failed to detected 5.1 for this certain file, but I don't think it is the case. Here are my tests for the true AVI with AC3:
Duration: 01:31:54.2, start: 0.000000, bitrate: 2124 kb/s
Stream #0.0: Video: mpeg4, yuv420p, 672x368, 25.00 fps(r)
Stream #0.1: Audio: 0x2000, 48000 Hz, 5:1, 448 kb/s
Note that there is clear indication of "5:1", plus the bitrate is higher than in your sample.