Hi Sebastian, welcome to the Baserow community. Judging from the example code that you have provided, I think you’re trying to upload an image directly when updating/creating a row? This is unfortunately not possible. You must first upload your file, to the user files and after that it can be used in row. You might want to take a look at the documentation of these endpoints https://api.baserow.io/api/redoc/#operation/upload_file and https://api.baserow.io/api/redoc/#operation/upload_via_url. They describe how you can upload from disk or from a URL.
The name that’s in the response of user file endpoints, is that name that you can use when you want to update the file field value of a row. A small downside is that you can only upload files via the JWT authentication at this point. That means you can’t use token that you generated via the web-frontend for this. This is going to be changed in the future.