3x-ui/web/assets/js/axios-init.js

axios.defaults.headers.post['Content-Type'] = 'application/x-www-form-urlencoded; charset=UTF-8';
axios.defaults.headers.common['X-Requested-With'] = 'XMLHttpRequest';

axios.interceptors.request.use(
    config => {
        if (config.data instanceof FormData) {
            config.headers['Content-Type'] = 'multipart/form-data';
        } else {
            config.data = Qs.stringify(config.data, {
                arrayFormat: 'repeat',
            });
        }
        return config;
    },
    error => Promise.reject(error)
);