I had the same issue on Steam but I managed to get it to work! Run the launcher one to get your settings in place but then close it don't run the game. Do the swap over from .zhe to .big (I ended up using a command prompt to save time) then run Zero Hour directly in Steam. For me this launched the game with the mod without running the launcher which seems to be the thing that kept reverting the file type. Hope this helps you too!
leezer
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