tonybradley,
From what I can see though the current email and antivirus software just isn't set up for that kind of system. They are set up to work on the blacklist system.
I don't know where you get this idea from? I know M$ OLEXP with the current patched/patches has some pretty harsh security settings that basicly work the way you mention.. pitty it don't allow you to sett up a "Whitelist or a Blacklist" or allow files to be released..

you can reject all attachments, and only view your emails in text only mode..ie no html.. that is the safest way to prevent virii.. As Louie said.. Someone could send you an infected file in a "safe" format..
While file type is one way to guard against worm, trojans, and virii. It is only a Frontline approach for an observant user, Heuristics/patterns would be the more effective method currently in use.. I think used by about 100% of the AV companies (Some one correct me please)

So if you are super paranoid.. reject all attachments and use a text based email client.
otherwise use a Good Antivirus programm that scanns your incomming emails and attachments. The effectiveness of these varies from Company to company and version (ie home or corporate), how upto date the definitions are, the end user settings.. like exclusions or the killer "turned the av prog off to install something and forgotten to turn it back on.."

Cheers