The nested item payload; required when '@odata.type' is itemAttachment.
namestring
Required
The display name of the attachment.
user_idstring
The user's email address or 'me' for the authenticated user.
event_idstring
Required
The unique opaque identifier of the calendar event (NOT an email address). This is typically a long string starting with 'AAMkA' or 'AQMkA'. Obtain it from listing or searching calendar events.