It seems the P1 can't play WMV files which use the WMV7 or WMV8 codec, only WMV9.
I have many older home movies using these codecs which come up as unsupported format.
This bug was in the 01.05.197 firmware and is still there in .232 and .235.
Here are some sample files to show the problem
Only the last one plays in the wiz
test_WMV1_V7.wmv
test_WMV2_V8.wmv
test_WMV3_V9.wmv