Are there any plans from marketo side to allow attachments in emails?
You could ask your support rep about whether this is on the roadmap.
Remember that a lead detaching and reading an attachment is not something you can detect, unlike following a link to content (gated or ungated). From a nurturing standpoint it hides important data. After all, we already try to drive clicks because opens are not a reliable metric. Attachments are sort of doubling-down on opens.