Holy cow, I wrote a book!
A customer observed that when they opened a Zip file containing
an Excel spreadsheet saved as XML,
the icon for the spreadsheet in the Zip folder
is just a plain XML icon rather than a fancy
"Is there any way to invoke a shell icon handler on an item
inside a Zip folder?"
Even if there were a way, you wouldn't like it.
Think about it:
In order to determine whether the XML file should get a
plain-XML icon or an Excel-XML icon, the Office icon handler
needs to open the XML file and sniff around to see if
has whatever it is that makes an XML file an Excel-XML file.
This means that the Zip folder has to extract the file
so that the icon handler can sniff it.
This means that opening a Zip folder would result in
decompressing every file in it just so that
it can give the decompressed file to the icon handler
so the icon handler can say what icon to show.
You probably wouldn't like that.
Therefore, Zip folders do not use icon handlers to obtain
icons for items inside Zip files.
uses the generic icon for the file extension.