gecko-dev/dom/html/test/formSubmission_chrome.js

var { classes: Cc, interfaces: Ci, utils: Cu } = Components;
Cu.importGlobalProperties(["File"]);

addMessageListener("files.open", function (message) {
  let list = [];
  let promises = [];
  for (let path of message) {
    promises.push(File.createFromFileName(path).then(file => {
      list.push(file);
    }));
  }

  Promise.all(promises).then(() => {
    sendAsyncMessage("files.opened", list);
  });
});