Hi Bruce,
For some reason I did not see the email saying your responded. Sorry for the slow response.
The code did not work. Nothing is being saved.
Couple of items that may change your code.
Your plugin is entitled cf7-database not cf7-db so the folder path is cf7-database/frontend.
Also, the data in the database only lists the file name but I figure that is all related.
Another item that you may considering in your next update. We have hidden fields because they are too long and want to be able to easily see all the form submissions without having to scroll alot (we will have a few hundred). However, when you export only visible fields are exporting. Having an option to export all fields no matter what fields are visible in the dashboard would be nice as we always want to export all fields.