Hi @mpugaf, great question! 
There are a few ways that we can do this. I recommend creating event handlers for our submit button.
For example,
If we have a "File Input" and a submit "Button," we can add two click event handlers to our submit button. One of them will be to show a notification to the user if the file did not have the correct prefix, and the other one to run the query that uploads the photo if it did.
​
Submit Button settings on the inspector.
This first event handler is to notify our users they need the right prefix on their file:
This second event handler is to run the query that uploads to our S3 bucket:
This way, our users can only upload a file it has the prefix we require, thus keeping bucket organized. 
Let us know if you have any questions!
​