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? Warning in Excel

When you export data to Excel and you try to open it with Excel 2007 and later, you will get the following warning:

The file you are trying to open 'TestData.xls', 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 click on Yes, the file opens and display the right data. But this warning is annoying to the user and can sometimes be misleading

This is an issue with Excel itself. Follow these steps to fix it:

1- Open your Registry (Start -> Run -> regedit.exe)
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- Type “ExtensionHardening” as the name (without the quotes)
5- Verify that the data has the value “0″ 

There is another way to do the same fix:
http://support.microsoft.com/kb/948615