Conventional file servers are a combination of DAS and NAS, rolled in to one. It is the network operating system (eg Windows Server, Linux, etc) that allows the contents of the server’s DAS to be made available and shared to the users of the network.
Physically, they are DAS devices. Logically, they are NAS devices. However, it should be noted that conventional network file servers provide far more services to the network than file sharing, whereas a true NAS device would only provide file sharing.