Invoice Attachments

You can now attach files to invoices, list them and remove them through the API. Attachments are
included when Rillet emails the invoice, the same as files attached in the Rillet application.

• POST /invoices/{invoice_id}/documents uploads one PDF, JPEG, PNG or XLSX file as
  multipart/form-data in the file field and returns the new attachment.
• GET /invoices/{invoice_id}/documents lists an invoice's attachments, oldest first, with a
  download URL that is valid for up to 1 hour.
• DELETE /invoices/{invoice_id}/documents/{document_id} removes an attachment from the invoice.

An invoice can have at most 10 attachments totalling at most 7 MiB, counting files attached in the
Rillet application. Attach files before the invoice is sent: changing attachments afterwards does not
change the email that already went out.

Applies to V4 only.