Hi,
Based on my knowledge,
Content Type specifies the content type of HTTP response and its contained in the URL access-type. If you do not specify a
Content Type, it defaults to text/HTML.
For an existing connection where
HTTP PAYLOAD is carried, if you add a new rule
to allow access only for specific content types, the request for the earlier allowed content type will be denied and the connection will be terminated.
When client (access external site via
TMG) requests a new content type in an existing HTTP connection, the
TMG Policy Enforcement of existing connections uses the content type which is derived from the requested
URL path to match the policy rather than renegotiating the establishment of the connection with the Web server.
The links bellow maybe helpful to better understand
TMG Policy Enforcement:
About policy enforcement
http://technet.microsoft.com/en-us/library/cc441731.aspx
TMG New Features - Policy Enforcement
http://blogs.technet.com/b/yuridiogenes/archive/2008/06/08/tmg-new-features-policy-enforcement.aspx
I hope this helps!