get uploadProgress() {
        let percent;
        if (this[kBodySize]) {
            percent = this[kUploadedSize] / this[kBodySize];
        }
        else if (this[kBodySize] === this[kUploadedSize]) {
            percent = 1;
        }
        else {
            percent = 0;
        }
        return {
            percent,
            transferred: this[kUploadedSize],
            total: this[kBodySize]
        };
    }
Progress event for uploading (sending a request).