@MalteKupzok When a file is uploaded through a form, the contact property will only return a signed URL. That URL can be used to download the document, but it’s time-limited for security reasons, so you need to fetch it and process the file while the link is still valid.
These files don’t appear in the Files API because they’re stored differently, but the signed URL is the correct way to access them. A common approach is to have your automation call the contact property, grab the signed URL, immediately download the file contents, and then move it into your own storage or processing pipeline.