Encrypting email attachments with mail policy rules

Hello,

We have several mail policys setup to automatically encrypt email if certain rules are fired...i.e. credit card or social number in the "body of the email".  My question is when attachments are sent with this information (credit card or social #) and the email body does not contain these items, the eamil is sent unencrypted.  I thought PGP would catch and encrypt these attachments.  Is there a rule/policy I need to configure for this?  Or do all attachments need to be PGP zipped before sending.  Thanks, Caribou.