application/x-pkcs12

The identity bundle: a password-protected archive holding a private key together with its certificate chain — what you get as a .p12 or .pfx when exporting a client certificate or a signing identity. The x- prefix is the tell: this is the Netscape-era unregistered spelling, still what most servers send even though IANA registered the clean application/pkcs12 long ago. Handle it like the secret it contains, and beware old exports — legacy PKCS #12 files encrypt with weak ciphers that modern tooling re-exports as AES.

.pfx is the older Microsoft name, .p12 the generic one — same format, no difference in content.

Defined by RFC 7292. Registry facts from the IANA media-types registry via mime-db.