Yeah, looking in it as text confirms its a webp. Just an fyi, lots of OSs/photo viewer things just kinda... try to interpret the file in different common formats if it fails to load in whatever format the extension says it is. My OS is pretty ancient (32-bit W7) so it probably doesnt even know what a webp is and fails to fallback properly, while yours is newer and maybe can handle it better?? idk, just glad I figured it out!
Suuuper excited about the jam, and heres to hoping we can all make some great games! :D