How can I set a Request's Attachment 'Part' property in a Groovy?

Hi.  This is so silly.  I am using Ready API.  Using Groovy Script, I am adding a file as an attachment to a request.  Everything is great, except the attachment 'Part' is missing its cid value.  If you look at the request and click the 'Attachments' tab, the file is listed but the 'Part' column is empty.  If I click into it, the cid is available in the drop-down.  And the cid is in the request.  How do I set the attachment's 'Part' property?

Thank you,

Angie