Cannot read properties of undefined (reading buffer) when uploading image

I am trying to upload a local image from flutterflow to firebase by calling the "Upload File to Firebase Storage (Authenticated)" API in buildship. For the API I use a Multipart body and I assign the "uploaded local file" value to the "image" variable which is also of type "uploaded file". It seems though, as if the image doesn't get send or has the wrong format (even though it is a jpg image). Any idea what I might do wrong here?