PGP: How to encrypt/decrypt attachment?

[HervE]

Hello forum,

I've told here about my advances with PGP.
Like I said, I've just managed to encrypt/decrypt a message to/from a customer.

But also sometimes (most of the time) the customer sends his message clear. Only the attachment is encrypted with PGP (it comes with .pgp extension - or .gpg)
How can OTRS (automatically) decrypt it, using my secret key?

In the other direction, how can I send a clear message, with attachment encrypted using the customer's public key?

Regards,
HervE