With the Preview Pane showing, if you select a file in the list, the Preview Pane will occasionally show "Preview not available". However, most of the time, the Preview Pane will show this error message:
When you click on an Excel file, to see the preview, an instance of Excel opens in the background. You can see that Excel instance, if you open the Task Manager, and check the list of Background Processes.
The reason these shortcuts include a 2 and a 6 is because they are second and sixth in the list of view options available. A shortcut can be made for any of the other options by using the corresponding number.