Problem
User opens an xls file in excel 2010 and the following error appears
"cannot open file because the file format or file extension is not valid."
Fix
We managed to solve this issue in our domain by relaxing the group policy setting forForce file extension to match file type
This can be accessed via User configuration\Administrative templates\Microsoft Excel 2010)\Excel options\Security
The options are listed below
- Allow different
- Allow different but warn
- Always match file type
We set this to Allow different which then allowed the user to open the .XLS file in excel 2010
If you disable or do not configure this policy setting, if users attempt to open files with the wrong extension, Excel opens the file and displays a warning that the file type is not what Excel expected.