For performance reasons, ATG Campaign Optimizer checks for page substitutions only on certain requests. The pages checked for requests are controlled by the substituterMimeTypes property in /atg/abtest/substituter/PageSubstituter. The default setting is:

substituterMimeTypes=dynamo-internal/html,text/html

If you have other mime types that you want to alter with ATG Campaign Optimizer tests, modify this setting to include the additional mime types.