Export issues: After doing an export to “Excel (All fields)” / “Excel (current fields)”, an excel file is generating. Attempting to open the file throws the following error about file format not matching its extension.

Microsoft Excel: "The file you are trying to open 'xxx.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?" Yes/No/Help

it still opens ok, but I'm not sure why the error shows up.