You may also try a different way of exporting via updating the data widgets module and using the javascript action to export to excel from a data grid 2 widget.
Mendix Marketplace - Data Widgets
For me, .xls files could not import, but .xlsx files imported fine, even though both file types are supposed to be supported.
If you're concerned about jar files, you might consider deleting all the jar files and then redownloading all of the marketplace modules in the project. Make sure everything still works after this.
Another option for queries to consider is to download a backup of the database from the production site, open the backup in postgresql and then run the queries there.