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X.509 public key certificate in DER or PEM form, read by TLS libraries

Handled by macOS itself

Format facts

MIME type: application/x-x509-ca-cert
Apple UTI: public.x509-certificate
Stock Quick Look: macOS renders a preview
Planned approach (census): native-sufficient
Icon hint: unknown
Prevalence: universal

Renderer note (from the census)

macOS QL shows the certificate summary natively

Gotchas: usually DER