FedEx REST API - Upload Document error (multipart stream ended)

I am testing the FedEx REST API via Python. I am able to perform the authentication and e.g. validate a shipment - however, I am unable to perform an Upload Documents POST request. The official FedEx docs unfortunately seem to lack a correct example for this (as of 21.02.2023)

My code is below:

url = "https://documentapitest.prod.fedex.com/sandbox/documents/v1/etds/upload"f = open("file.txt", "rb")text_data = f.read()f.close()headers = {"Content-Type": "multipart/form-data","x-customer-transaction-id": "12XXXXXX","Authorization": f"Bearer {access_token}",}response = requests.post(    url,    files=dict(        document='{"workflowName":"ETDPreshipment","name":"file.txt","contentType":"text/plain","meta":{"shipDocumentType":"COMMERCIAL_INVOICE","originCountryCode":"DK","destinationCountryCode":"BE"}',        attachment=("file.txt", text_data, "text/plain")    ),    headers=headers,)print(response.text)

I get an error code 503 with the response.text providing the following error:

 {  "customerTransactionId": "12XXXXXX","errors":[           {  "code":"SERVICE.NOT.AVAILABLE","message":"Enterprise Document Upload Service Unavailable"         }      ]}

I am unsure if this indicates an error in my script, or if the 'Upload Documents' API is only supported for production environments?