How to attach a file to a sublist via SuiteScript?

I would like to attach a file to a sublist, specifically the Expense Line in the Expense Report. I have used record.attach to attach a file to a Vendor Bill but I cannot see a way to attach files to the Expense Report Lines. I used record.attach for the Vendor Bill.

var id = record.attach({
    record: {
        type: 'file',
        id: '200'
    },
    to: {
        type: 'customer',
        id: '90'
    }
});

Any guidance much appreciated.