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.

How to Recover a Microsoft Excel File That Is Not in a Recognizable Format. If Microsoft Excel fails to recognize a file in the XLSX or XLS format, the data contained in the file could be corrupted.