Developing file type extensions for custom XML file types

Is it possible to develop an extension with a pre- and post-tweaker class for a custom XML file type?

I got it to work for the standard Any XML file type, the XLIFF file type, Word, etc., but not for a custom XML file type created through the Studio GUI.

My extension plug-in just does not get picked up by the file type and appears nowhere in the file type list, although the plug-in is recognized when staring up Studio.

Thank you

Ziad