gecko-dev/dom/file/tests/create_file_objects.js

Cu.importGlobalProperties(['File']);

addMessageListener("create-file-objects", function(message) {
  let files = []
  let promises = [];
  for (fileName of message.fileNames) {
    promises.push(File.createFromFileName(fileName).then(function(file) {
      files.push(file);
    }));
  }

  Promise.all(promises).then(function() {
    sendAsyncMessage("created-file-objects", files);
  });
});