In 8.6 SP1 and 9.0 SP2 you may see an issue where a form export creates a file that gives an error upon opening in Excel 2010 or 2013
Excel 2013 Error: 'form_data_export-1.xls' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway?
Excel 2010 Error: The file you are trying to open, ‘nameoffile.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.
Here are the steps to resolve this issue.
Rename the extension to .html.
You'll need to open the file from Excel or it will open in your browser.
Alternatively, you can download and install the latest CU from /downloads which should permanently change the file extension to .html.