sniff webp from the magic number
sniff webp from the magic number

removes the 100 byte minimum file size

see https://github.com/lovell/sharp/issues/996

jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,lovell/libvips