function main() {
  var label = 'Clients/New York/Design';
  var folder = 'design-assets';

  folder = DriveApp.createFolder(folder);

  GmailApp.getUserLabelByName(label).getThreads().map(lastMessage).map(extractAttachment).forEach(saveFile(folder));
}

function lastMessage(thread) {
  return thread.getMessages().pop();
}

function extractAttachment(message) {
  var blob = message.getAttachments()[0].copyBlob();
  var filename = message.getFrom() + '.' + extension(blob.getName());
  blob.setName(filename);
  return blob;
}

function extension(filename) {
  return filename.split('.').pop();
}

function saveFile(folder) {
  return function (blob) {
    folder.createFile(blob);
    Logger.log('Saved file ' + blob.getName());
  };
}