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.

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.

Learn how to change the default file format for saves in Word, Excel, and PowerPoint, so that you do not need to change it every time.

Opening XLSM Files in Excel By default, you should be able to open an XLSM file without an issue (as long as you're using Excel 2007 or later), but the macros normally aren't enabled by default.