monitoring for excel attachments

Hi everyone

OK, I need to set a rule to monitor for excel attachments being sent out by users.

I've tried creating rule:

          any attachment's file extension matches:         *.xlsx  *.*xlsx  xlsx  *xlsx  

           generate a report and send it to     Me.

but when I send an email with an excel attached, from outside, to my internal mailbox, it's not picked up. it doesn't generate a report.

does anyone see an error or know what's the correct word or phrase to use for excel documents?

thanks

G