You can create the rule with SMTP resource. Under SMTP check Action2 tab. You can use "Strip file by name"
Quote:
| Strip file by name strips file attachments of the specified name from the message. This field enables the user to strip UU-ENCODE and MIME file attachments whose names match any of the defined expressions. Consider the three expressions: {+love*, *.pic, a*.+}. In the following examples, the defined file attachments will be stripped |