Here is one tedious solution.
1.
Download[
^] IECapt. (please read the document, to see how to use this)
2. Create a webpage where you pass document path, as a query string and it will display that document. (you need to use office automation to show doc / xls file)
3. now call the IECapt exe (using System.Diagnostics.Process class), using your webpage. and pass the url to the page you have previously created with specific document path as query string.
4. this will store the image of the webpage in specified folder.
5. now read that image and display as thumbnail.
I know this is messy solution, but all the other easy methods are not free, unless you want to buy those controls.
:)