Microsoft Excel 2007 error - Trying to open a different format than specified by the file extension

On Microsoft Excel 2007, you may get the following error when you try to open an Excel document:

A quick and dirty but easiest way to solve this problem, is to insert the following registry key. This will simply suppress the notification:

[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security]
“ExtensionHardening”=dword:00000000

The step-by-step instructions are as follows:

1. Click Start, click Run, type regedit.exe and press ENTER. This will open your Registry Editor.
2. Navigate to HKEY_CURRENT_USER\SOFTWARE\MICROSOFT\OFFICE\12.0\EXCEL\SECURITY
3. Right click in the right window and choose New → DWORD
4. In the Name field, type ExtensionHardening.
5. Verify that the data has the value 0.