.ico

Windows icon resource, the multi-image format used for application and website icons on Windows

Handled by macOS itself

Format facts

MIME type: image/vnd.microsoft.icon
Apple UTI: com.microsoft.ico
Stock Quick Look: no stock preview
Planned approach (census): native-sufficient
Icon hint: renders
Prevalence: universal

Renderer note (from the census)

macOS renders ICO natively