If you have a small valid webp image handy, could you possibly also add this to this PR into tests/images/webp or something? Seems like we currently lack any samples.

Also, I think in the long run this may not be an optimal solution as RIFF is a container format. However I much prefer header based format detection than full blown format parsers, but again we may have to.

For example TGA cannot simply be detected with magic bytes, one would need to parse more of the file to conclude its valid TGA.

I agree, but I think that would be for a later PR. For now webp is the only image format supported by this library where the file can start with RIFF so I assume this should be safe for now.

Yea, for sure. The only other format using RIFF container I'm aware of that's even remotely resembling our use case is Windows 95 animated cursors (.ANI).