Uploading file via REST API/Python

Hi, I am working on an item that uploads .csv file into Anaplan. I'm not sure why the upload doesn't work even if the response of API is successful. I've tried using some codes posted here but seems the upload does not work (download, import seems okay)

I'm using basic authentication and 1.3 version of the API.

Here is a snippet of the code

    user = AnaplanUtils.create_auth_session()
    file_id = '123'
    file_name = 'test.csv'
    url_file = api_url + 'files/'
    url_complete = url_file + "/complete"

    file_metadata_start = {
        "id": file_id,
        "chunkCount": -1
    }
    file_metadata_complete = {
        "id": file_id,
        "chunkCount": -1
    }

    start_upload_post = requests.post(url_file, headers=build_headers(user, 'application/json')
                                      , json=file_metadata_start)

    file_data = open(file_name, "r").read().encode('utf-8')

    if start_upload_post.ok:
        file_upload = requests.put(url_file,
                                   headers=build_headers(user, 'application/octet-stream'), data=file_data)
        if file_upload.ok:
            complete_upload = requests.post(url_complete, headers=build_headers(user, 'application/json')
                                            , json=file_metadata_complete)
            if complete_upload.ok:
                print('success')
            else:
                print('There was an issue with your file upload: '
                      + str(complete_upload.status_code))
        else:
            print('There was an issue with your file upload: '
                  + str(file_upload.status_code))