Excel 2007 user warning when opening xls files

Question

I am getting the following error warning when opening an .xls file generated using 'CarlosAg.ExcelXmlWriter.dll' in Excel 2007:

The file you are trying to open,'', 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?

Any ideas how I can resolve this?

Answer 1

There's a Microsoft discussion of the issue which provides some mechanisms to resolve:

http://blogs.msdn.com/b/vsofficedeveloper/archive/2008/03/11/excel-2007-extension-warning.aspx

Answer 2

It seems that the .xls file (Microsoft Excel 97-2003 format) is actually an .xlsx file (Microsoft Excel 2007 format). Changing the file extension from .xls to .xlsx should clear the warning message.