Well, there's no way to attach files within the Marketo platform. Only with a very complex multitier setup* can you make this work.
But I’m not sure what you mean by “success rate” here. Almost by definition, you can’t know how many people open attachments. While you can know how many people open links. Can you please clarify the exact type of document you’re looking to attach and what you want to report on?
* For one and only one Marketo instance, we use an intermediate SMTP gateway to add attachments. Marketo sends (everything) via the gateway, which allows us to pick up certain instructions in the email body and add attachments. BUT the instance already used that gateway for compliance reasons, so it was a comparatively small lift. If you don't already do that, it’s very big job.
