JCasC cannot easily configure UploadedKeyStoreSource as the expected input is a bytes array.
Users would like to configure the certificate via the same "upload" function that is provided by stapler when configured via JCasC

https://github.com/jenkinsci/credentials-plugin/blob/bddcf41ba3388cb54126e3f626b3deb7d65e3b53/src/main/java/com/cloudbees/plugins/credentials/impl/CertificateCredentialsImpl.java#L677

Workaround: "cat file.pkcs | base64"