Error opening file: "The file format differs from the format that the file name extension specifies"

You export the data from GridView to Excel file and when you try to open the file you get the following warning message:
The file you are trying to open, 'Filename.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 this file. Do you want to open the file now?
How to solve this issue???