message: explain the feature is chrome only
diff: // under the License.
  import browser from "webextension-polyfill";
  +import parser from "ua-parser-js";
  import { recorder } from "./editor";
  import Debugger from "../debugger";
  +const parsedUA = parser(window.navigator.userAgent);
  +
  export default class ExtCommand {
  constructor(contentWindowId) {
  this.playingTabNames = {};
  @@ -230,6 +233,8 @@ export default class ExtCommand {
  doType(locator, value) {
  if (/^\//.test(value)) {
  + const browserName = parsedUA.browser.name;
  + if (browserName !== "Chrome") return Promise.reject(new Error("File uploading is only support in Chrome at this time"));
  const connection = new Debugger(this.currentPlayingTabId);
  return connection.attach().then(() => (
  connection.getDocument().then(docNode => (
  