Note: This kind of proxy is largely an ideal. Implementations will fall short of this goal. AI research may have to catch up a bit first for all these ideas to work.
This kind of proxy is aware of the content type as before, but also possess abilities to decode and reject the content of a transmission for reasons of local policy. This would include not only security policy, but other things, perhaps including local sexual harassement policy.
The most probable case is scanning for viruses. A less ambitious HTTP proxy might search the HTML content of the download for Java code and remove it if the local policy says no Java from the net.
This case is distinguished from the previous case in that the content of the transmission is examined, while the previous case examines only the packaging. This case might recognize a MS-Word document that appears to need an X rating, while the previous case would simply remove any MSWord virus macros.