Do you have a plan that files will able to download file via REST API ?

You can try setting up a custom PHP Thank You page to receive the data from Jotform, and make your own API call to the external service from that PHP page.

Please take a look at the guide on Sending-POST-Data-From-JotForm-Using-PHP-in-Custom-Thank-You-Page. You will then use this data in your custom script to perform the custom function that you are after (your API call), in a manner similar to how-we-can-send-Submissions-to-Your-MySQL-Database-Using-PHP.

Additionally, please check our API documentation: http://api.jotform.com/docs/

Thank you!