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.