XLS files are Excel workbook files created in Excel 97 or Excel 2003. The 2007, 2010 and 2013 versions of the program use the XLSX format by default to save workbooks, but can also use the XLS format.

1] Keep preview images for individual Office documents Open the file (Excel, PowerPoint, Word) for which you would like to add a thumbnail Microsoft has named the feature as Thumbnail and not Preview.