Upload a file on macOS with NSURLSession

I'm trying to upload a file on macOS using an NSURLSessionUploadTask but keep running against a wall. Even though all seems to be fine and set according to specs I keep getting an Http status 400 "API upload did not contain a file part".

The curl example from the api docs works perfectly so I assume there is some incompatibility as to how NSURLSession uploads the files as part of a multipart/form-data POST request which the API endpoint doesn't seem to support.

I use basically the same code for uploading files to Google Drive and it works just fine there, so I don't think it's an error on my part.

Any help would be greatly appreciated...

headers as returned by httpbin.org:

"form": {

    "attributes": "{\"name\":\"localhost.zip\", \"parent\":{\"id\":\"<folderid>\"}}"

  }, 

  "headers": {

    "Accept": "*/*", 

    "Authorization": "Bearer <token>", 

    "Content-Length": "1569178", 

    "Content-Type": "multipart/form-data; boundary=------------------------52ac5dd531376fde", 

    "Host": "httpbin.org", 

    "User-Agent": "curl/7.79.1", 

  },