When present, contains the ID of an external attachment that can be retrieved in a separate messages.attachments.get request. When not present, the entire content of the message part body is contained in the data field.
Number of bytes for the message part data (encoding notwithstanding).
The body data of a MIME message part as a base64url encoded string. May be empty for MIME container types that have no message body or when the body data is sent as a separate attachment. An attachment ID is present if the body data is contained in a separate attachment.