Attachment blocking policy question

I have created an outgoing policy to check the outgoing attachment. The logic is that:

1. If there are no attachment in the email, then deliver it, otherwise

2. if the attachment is in format of OUT_*.ZIP, then deliver it, otherwise deny it.

The problem is this. when sending an Encrypt and Signed email, it will trigger the rule 2 and block it even there are no attachment.

The question is that why Encrypt and Signed plain text email will detected  as an attachment, how can I check that the outgoing email is Encrypt and Signed, and bypass it.