Blocking Attachments

Hi there everyone,

I want to block certain attachments coming in via e-mail. My setup is: SBS 2000 with Exchange2000 and ISA server. All 3 on the same computer. All SPs are installed.
I would like to stop all e-mails with attachments that havethe following extensions: *.exe and *.scr. Could someone please let me know where I can set Exchange up to stop those mails. I am having a serious problem with attachments like that.

Thank you all