Are you sure that it's Norton that's interfering with your viewing of attachments?
The e-mail security upgrades that have been a part of the recent Office (and Outlook) service packs do precisely what you are referring to, and they do so by design.
If you have installed the most recent Office service packs, they have available a means by which you can allow attachments with certain file extensions. You can read a bit more about this here:
However, before you make adjustments which allow new attachment types to enter your system, you should think long and hard about this. Even though these attachments may come from friends, hackers, virus kiddies and spammers can get hold of your e-mail address and send you things too.
I have found the default security levels for e-mail attachments in Outlook quite appropriate....I can still send and receive most image and document files. Things like .exe files are blocked but you shouldn't send or receive those anyway, even if it is from a friend...it's too easy to make a mistake that can be very difficult to fix.