Unfortunately it's all down to the changes that Microsoft has made in Outllib.dll in the security packs. This dll lists all the file extensions that are level 1 (can't be relaxed) and level 2 (can be relaxed).
However, I don't belive that it's possible to relax teh rules under Office 2000, only Office XP.
If you really want to fix it, you'll need to hex-edit the outllib.dll file and simply overtype the values that you want to allow as attachments.
The file address for the attachment list inside outllib.dll is :1201136.