Attachments work pretty badly in the Invoices & Estimates, to be honest.
Sometimes if you attach one, it removes the file extension in the email, causing it to:
  1. Not be downloadable / viewable
  2. Blocked by some email clients
Additionally, there is no way of viewing or downloading the attachments from the Estimate/Invoice window. Meaning that if you're only sending the Estimate / Invoice URL (or sending via SMS) there is no way of sharing the attachments too.
Additionally, by default we should be able to attach a PDF version of the estimate/invoice to the email notification that is sent. This would allow clients to BCC their accounting software or ERP to the email in if the integration doesn't already exist with GHL, as well as for simplicity for customers. This should also be doable through workflows.