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 you see Sorry, Photos can't open this file because the format is unsupported or the file is corrupted error in Windows Photos app, this guide will help you.

Choose "UTF-8" from the drop-down box next to "Encoding" and click "Save." Your text file will be converted and saved in the UTF-8 format, although the file extension will remain the same.