Nessus File upload REST API

I'm trying to upload an exported scan (.nessus) file to a Nessus Community Edition server using python and the Nessus REST API (func POST /file/upload) however I keep getting the response null like this {"fileuploaded":null} in the response.

I can't seem to see in the API doc's what else could be required.

def upload_scan_file(_path):
    _url = url+"/file/upload"
    _head['Content-type'] = ''
    _files = {"file": open(_path, 'rb'), "no_enc" : "0"}
    r = requests.post(_url, headers=_head, verify=False, files=_files)
    return r.text

The reason I unset the Content-type key in the headers dict is that I get a {'error': Content-type: application/json not supported'}

_path contains the file path.

_head is a dict of header values that I use to query all the others information.

Any help would be appreciated.