Calling included templates

How would one go about modifying templates that are called using <xen:include ...>

I tried the code events: create template & post render... and they both do not see those included templates being called.

Any help would be appreciated.
There's no hook in the template but it's: attached_file ... what I basically want to do is display certain extensions differently, but I want to do it without editing the template... if there was a hook, it would've been easier (or if it was called in a template code event, unless I missed something)
I'm new to using this system, and a bit leary about template edits also , so another thing I've been trying is going back to the nearest existing hook, creating my own template within the hook and assigning it to my own add-on. This way I can replace template sections leaving the default template intact.
Yes. But there are no hooks in attached_files. I could've used a dirty hack to html replace it on template create but template create does not show the template being created.