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.

Sometimes, when you try to open a Word file it fails to open and displays the following message – Word cannot open the file because the file format does not match the file extension.

Red Eye Rose received a file with a strange extension. She doesn't know into what program she should load it. Neither does Windows.

Normally, if you change a file’s extension in Windows, it doesn’t do anything positive. It just makes the file open in the wrong programs that can’t decode what’s inside. Ho… ...