A separate KB article describes allowing of C/C++ files with a non-standard extension. For programming languages other than C/C++, e.g. XML, you need to add your custom extension to a single registry key to activate Visual Assist on the non-standard extension.
Exit the IDE and use regedit to find the key that describes your programming language. For example:
HKEY_CURRENT_USER\Software\Whole Tomato\Visual Assist X\<IDE spec>\ExtXml = ".xml;"
Several different languages will be available. Locate the language name you want after "Ext" in the key name. If you see more than one key, e.g. "ExtXml", "ExtXml2", "ExtXml3", add your extension to the highest-numbered key.
Features of Visual Assist that become available include: