A file attached to the message.
Fields
filename(required)content(required)content_type(optional, nullable): The MIME type of the attachment. Supports parameters (e.g.text/calendar; method=REQUEST). If omitted, detected automatically from content or filename.content_id(optional, nullable): Content ID for inline attachments, referenced viacid:in HTML body. If no@is present,@lmis appended automatically.
Absent fields are :unset; nil is JSON null. extra holds response fields that this version of the SDK does not know.