Unable to view some email attachments

I am running Windows 2000 environment on three servers DC1, DC2 and EX1 (Exchange 2000) and the email client is Outlook 2000. My users are complaining about not being able to view certain attachments. I looked under the help on Outlook and it explains that these files are being blocked under a level 1 security. It goes on further to say that if you use Microsoft Exchange Server, your administrator can add and remove file types for both levels of e-mail security.

My question is how and where do I do this in Exchange 2000? I just recently migrated from Exchange 5.5 to 2000, still learning the in and outs of Exchange 2000.

Thanks in advance for your help.