http_sub_module, need help

[raptium]

I am using nginx as a reverse proxy for IBM Lotus Domino Web Mail.
Since there are some internal only URL in email content, I need
http_sub_module to modify(add some prefix ) the URL so that user
outside intranet can click the link and access through another portal.

I add

   sub_filter aaa bbb;
   sub_filter_types application/json application/x-javascript;
   sub_filter_once off;

into the nginx.conf and it does not work.'aaa' is not replaced by 'bbb'

However, if I save the web page as html file or generate some json
which contains 'aaa'. It can be replace by 'bbb'.

Therefore, it there any other options besides `Content-Type` can
prevent http_sub_module replacing the word?

Sorry for my bad English...