Attachment File Type List

I have just recently installed Microsoft Exchange Server on my network. I understand there is way to add and remove file types from the attachment security list but can not find how to do this. I need to delete a file type from this list. I have been sent an Access database update that I know is virus free and cannot open it as it is listed as a level 1 security type. Any assistance is always appreciated. Thank you. p_Jacobs@wch.org