Xero: Ability to add attachments

I enjoy using workato for creating invoices and bills on Xero from data gathered by dataparser. However, unlike other API integration sites (such as Zapier), I can't see the ability to add an attachment on to a bill or invoice - a vital feature for our work. I can add URL's, but this isn't really the same thing. If Zapier can do it, I'm sure Workato can, with the resources at your disposal.