How to create 'upload stream' for passing documents to applicant API

Hey there!

I have troubles sending documents to you applicant API from my Wordpress installation with the help of a forms plugin (Gravity Forms).

The plugin is able to send a POST request after form submission to an API endpoint.
This works for all text fields (like name, email, message, ..) but does not work for documents. They don't arrive in Personio.

I would be able to hook into into the process to mutate the request data before it is sent to Personio. The plugin authors advise to base64 encode the files before sending them inside the POST request but this also doesn't work with the Personio Applicant API it seems.

Could you advise on how to create this 'upload stream' your API documentation (https://developer.personio.de/docs/applications-endpoint) suggests from a PHP environment (Wordpress)?

Any help / hints much appreciated, thanks!
Timo