from flask_restx import Resource, fields
from flask import request
import sys
import os
from werkzeug.datastructures import FileStorage
from werkzeug.utils import secure_filename   

BASEDIR = os.path.abspath(os.path.dirname(__file__))

upload_parser = api.parser()
upload_parser.add_argument('file', location='files',
                               type=FileStorage, required=True) 

@api.route("/your_route_name")
@api.expect(upload_parser)
class MyResource(Resource):
    @api.marshal_with(<output_dataformat_schema_defined_here>, skip_none=True)
    def post(self):
       uploaded_file = request.files['file']

       if uploaded_file:
          secured_logfilename = secure_filename(uploaded_file.filename)
          uploaded_file.save(os.path.join(os.path.join(BASEDIR, 
             "current_logs"), secured_logfilename))
          return {"message": "File successfully uploaded"}, 200
       else:
          return {"message": "File upload unsuccessful"}, 400

Source: Flask restx, Flask