requests_toolbelt: 415 Client Error: Unsupported Media Type

Question

I am trying to upload file as follows:

from requests_toolbelt import MultipartEncoder
import requests

file_details = {'upload-type': 'gallary', 'username': 'a457'}
url = "url/to/upload"

with open('path/to/file', "rb") as file:
    encoder = MultipartEncoder(
        fields={
            "file": (
                'gallary.zip',
                file,
            ),
            **file_details,
        },
    )
    response = requests.post(url, data=encoder)

I get following error: 415 Client Error: Unsupported Media Type. The api requires json data to be submitted that is file_details should be json data. How can I fix this error?