Inside MyUploadAdapter

    _initListeners( resolve, reject ) {
        ......
        
        xhr.addEventListener( 'load', () => {
            const response = xhr.response;
            if ( !response || response.error ) {
                return reject( response && response.error ? response.error.message : genericErrorText );
            }

            // If the upload is successful, resolve the upload promise with an object containing
            // at least the "default" URL, pointing to the image on the server.
            resolve({
                default: response.s3Url
            });
        } );
        
        .....
    }