You can easily filter out sobig.f with postfix too. This can be done by adding "body_checks = regexp:/etc/postfix/body_checks" to main.cf, and creating a file named "body_checks" in /etc/postfix. Just add the following to the body_checks to accomplish filtering of .scr and .pif attachments:
/[TAB SPACE]name=.*\.pif/ REJECT This mail server does not accept this type of attachment
/[TABSPACE]name=.*\.scr/ REJECT This mail server does not accept this type of attachment
where TAB is a tab character and SPACE is a space character. This has worked well for me.