Currently there is no option to include the files to be sent in email to customer with object e.g. Invoice.

Attachments API has an option includeonline to specify if we want to send the attachments in the email to the customer.

Also there is an issue to include multiple files via the attachments API. I have added 2 files via the attachments API to invoice only one of them is showing as included online but other file need to be manually included via Xero GUI