var attachments = message.getAttachments();

"oauthScopes": [
  "https://www.googleapis.com/auth/gmail.addons.execute",
  "https://www.googleapis.com/auth/gmail.addons.current.message.metadata",
  "https://www.googleapis.com/auth/gmail.addons.current.message.action",
  "https://www.googleapis.com/auth/script.external_request",
  "https://www.googleapis.com/auth/script.locale"
]