once they asked me to do such a task to disable what so called webmail. or any other web service that you are able to upload or send files through http servers (uploading,attaching through firefox). so I found a way that you could that with denying any method=post on proxy server and it works great. so try it and it will help you if your company wants such a policy, and by the way you still browse the internet and check your emails but you can't send or attach files.

Greetings from Lebanon
Maroon Ibrahim