Suppose your thumbnail has gotten out of whack with your workbook. How do you regenerate it? Sure, the brute force way is to re-upload the file again, but surely there must be a geek’er way to do this. Fortunately there is: just edit one of the SharePoint properties for the file. I always select the “Title”. I delete the last character in the title of the file and then re-enter it. Click Save. Since this document property has to be back populated into the file, by changing a property forces the event handler to fire and the thumbnail is regenerated. There ya go!