Let me break down how the app works a bit. Firstly, I recommend using a web app since it's a less expensive option compared to workflows. 
The first query we can look at together gets the ids of all the files in your Retool Storage.
On success, I've set up an event handler that will call the javascript query below:
This query loops through the file ids and triggers a query called
deleteFile that accepts a file id we passed as additional scope and deletes it from Retool Storage.Here's the query for deleting a file. Be sure to add the key/variable you created using additionaScope in the Advanced settings section so that the variable is in scope to the query.
Hope all of the helps! Let me know if you have any roadblocks!