One of the problems we have is that we do not store client files locally on our servers, this means we need to pull the file down from our cloud storage provider, before attaching, this brings in a lot of extra latency.
One possible solution would be to add an optional token that can be embedded into your email body, perhaps something like @AttachedFileList@. This would then list out any attached files directly on the email, without requiring the user to go in to view the invoice.
Would that be a possible solution?