Actually you can. To find all Word documents, you could type *.doc (same as XP, actually).
Or, somewhat easier... just click the option you want. Here's some helpful links:
How to use XP-like filters when Searching in Windows 7
How to search by file extension with the * wildcard
How to search by file type like "document"
Note: in this last one, you don't have to type -- or even know -- the code to type. Just click the button and it adds the code for you.
If any page link above doesn't load correctly, press F5 or refresh the page to make it load. The site these pages are stored on is having difficulties.
Oddly, the search filter buttons are limited by how wide the search box is. If you drag the left edge of the search box to make it bigger, more options appear. I sure hope this is addressed in Windows 8.