I think you are wanting to use the web post protection rule.
You can define criteria for your files to be blocked when sent over the web in Internet Explorer.
For other browsers, use the network communication protection rule.
For internet explorer use web protection rules
1) create web defination abc.com'
2) during web protection rule uncheck abc.com and select other domains and select which files you need to restrict as an attachment.
you can replace abc.com with any site you want to allow sending attachments. In above rule all emails domains are blocked to send attachment via internet explorer.
For Other browsers use network protection rules.
I would try this out and give a feedback before the week runs out!!!