Enum MimeType

// Use MimeType enum to log the name of every Google Doc in the user's Drive.
const docs = DriveApp.getFilesByType(MimeType.GOOGLE_DOCS);
while (docs.hasNext()) {
  const doc = docs.next();
  Logger.log(doc.getName());
}

// Use plain string to log the size of every PNG in the user's Drive.
const pngs = DriveApp.getFilesByType('image/png');
while (pngs.hasNext()) {
  const png = pngs.next();
  Logger.log(png.getSize());
}