The first question is why are you even doing this? VS allows you to add any file type to any folder for good reasons.
You cannot modify the behavior of Add Existing Item. In the VS implementation, the items has already been "validated", because, well, you're allowed to add any item you want at any time to any folder.
The event you're using is also tagged
Quote:
This API supports the product infrastructure and is not intended to be used directly from your code.
Microsoft Internal Use Only.
There is no event that fires before the items are added, nor after they after they are selected.
If you're trying to add specific file types from the interface, you're going to have to supply your own menu item outside of "Add Existing Items", complete with your own OpenFile dialog and code to filter specific items.