Is this in very recent Office versions? Or are you talking about the XML format used by Office 2007 upwards? Because that does still use different file extensions for different programs, obtained by adding an 'x' on their old binary format extensions (.docx rather than .doc, .xlsx rather than .xls, ...). They're zip archives of XML files rather than individual XML files so .xml wouldn't make any sense.

I got the impression the problem in the comment above was too many extensions rather than too few. For example, you have use .docm etc rather than .docx if your document contains macros otherwise Word will refuse to open it (this is a security feature to prevent document viruses). But it sounds like there are many others.

I mean, this is only adding another entry to your array of extensions for the specific icon. It's also not a deal-braker if it doesn't work. It really is easy, I don't see what's the problem.