By default Office document like word and excel take the standard icon ( I mean the word icon which represent the word document) but if you want to get a cute little preview , like the images get and you can visualize what they look like before you can open it.
You can not see whats stored in document completely with this but it can tell if documents first page is filled in or not. The image on the right is exactly what I am talking about. I tried this with Microsoft Office 2007 version.
How do I save it as Thumbnail ?
Its simple. You should get this option either when you save it for the first time or when you do save as. Look for Save Thumbnail option there. As soon as you save it you would get the preview exactly like the image on the top.
If you see closely here, this is the right place even to add authors and tags to the file. You need not to hunt places for it. Just make it a habbit of tagging it.