application/pkcs12

A PKCS #12 bundle of keys and certificates, usually protected by a password.

Extensions
.p12 .pfx
Header
Content-Type: application/pkcs12
Registered
Yes, in the IANA media type registry.
References
IANA

What servers send

The type each server's own mime.types gives these extensions, as it ships.

ExtensionApache httpd 2.4.69nginx 1.31.6
.p12 application/x-pkcs12 Not listed
.pfx application/x-pkcs12 Not listed

Common application types

Every media type, in one table