Windows, Excel2010, Office
The file you are trying to open, filename, is in a different format than specified by the file extension. Verify that the file is not corrupted and is from a trusted source before opening the file. Do you want to open the file now?
If you are using Office 2007, please select "Yes" from the resulting dialog
Go to Start, input regedit at "search for programs and files" (for windows XP or vista user, go to start>run, input regedit and press enter)
Navigate to HKEY_CURRENT_USER\SOFTWARE\MICROSOFT\OFFICE\12.0\EXCEL\SECURITY
Go to menu Edit > New > DWORD
Name: ExtensionHardening, data: 0
Reboot your computer