It's currently not possible to add Attachments to a Prepayment via the API.

It'd be nice if we could access this at "https://api.xero.com/api.xro/2.0/Prepayments/{Guid}/Attachments/{Filename}" to bring the functionality in line with the GUI.

We currently have access to: Invoices, Receipts, Credit Notes, Repeating Invoices, Bank Transactions, Bank Transfers, Contacts, Accounts, Manual Journals, Purchase Orders, Quotes (https://developer.xero.com/documentation/api/accounting/attachments).

Seems like the standard and infrastructure is already there, it would be nice to have this one soon, thanks!