Searching in body of message

I've started using procmail. What I want to do is to select all
those messages which contain a file attached which is in msword
format. 
I've tried to put the following rule in .procmailrc:

:O B
* ^Content-Type.*msword
attachments

Unfortunately procmail doesn't seem to find the regexp, and indeed
it cannot find ANY expression which is located in the header of the mime
encoded part of the message body.
How can I solve this problem?
Thanks a lot

   Massimo