If you cannot open an Excel file and see Excel cannot open the file because the file format or extension is not valid error, see this post.

If Microsoft Excel fails to recognize a file in the XLSX or XLS format, the data contained in the file could be corrupted. When you open a corrupted workbook in Excel, the program should ...

How to Open an XLS File in Excel. 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 ...