You can use this feature for contracts, floor plans, moodboards, etc. and you are able to upload assets within projects in 2 ways:
Attach directly to a document (invoice or proposal). You will see the "Attachments" box on the right hand side (underneath the payment section) on the IN or PR. *Note that once this document has been sent ("published"), your client will have access to view these attachments
Add an attachment generally to a project. Within a project, you'll see the "Attachments" tab. If you click into this tab, you are able to add attachments just to the project (versus to a specific document) - these attachments will not be viewable by your client. This tab also shows you a list view of all the attachments associated with documents within the project, indicating which document each attachment is attached to, as well as if published or not (aka - if it is viewable by your client).
Add attachments from proposals to invoices. If you have attachments on a proposal, you'll get a pop-up option to move the attachment to the invoice.
Watch this tutorial to see how to use this feature.