The file you are trying to open…

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?

You may get this stupid error message whenever you are trying to open an Excel spreadsheet  generated from a gridview using ASP.NET

Apparently this is  a bug

When you think that you finally nailed it, then there is this stupid warning (BS) .

Thanks Microsoft.