To answer my question: Out of the box, MATE is not able to parse the public key data contained in a PEM file, nor any other file type.
In Properties Caja recognize the file type correctly, though.
I also tried to convert the public key file from PEM to XML <RSAKeyValue>, but that did not work either.
I did not find any application that would register itself for .pem files or other files used for public/private keys.
As a side note, it seems the file type is not determined only by the file extension, since the type shown for the private.pem file is X.509 Certificate (application/x-x509-ca-cert).
