I’m not sure why it isn’t working with your codecs (which are more up-to-date than mine).
I did get a response from the author of Sciter though, and they recommended trying to load a video format supported by Windows by default. So I wonder if a WMV (Windows Media Video) file would work for you?
FunWrite only recognizes videos with .MP4 extensions at the moment, so just rename the extension from .WMV to .MP4 to get it to show in the dropdown (I confirmed this works on my computer).