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 Convert DAT to Text in Excel. DAT files are files with an extension that isn't associated with a specific application or program and can be almost any type of file including video, media or ...

If your Excel files are opening in Notepad on Windows PC, here's how you can reset the settings to make .xls, .xlsx, etc files open in Excel.

Need to look at invisible files and show hidden file extensions in Windows? Here's how to reveal them.