Workfront has the ability to increase the storage amount. We ran out of storage in the last year and we upgraded our storage amount no problem.
That being said, if you purchased this feature it would not be considered by most people to be an internal server, typically an internal server is something that you have physical access to. We necessarily have documents for proofing and approval on Workfront but our artists do work off of a typical internal fileserver, something that an internal team has access to and manages.