To download a file associated to a Deliverable, we should use $value on the required FileType, as shown below:
GET
<ServiceRoot>/Sites('MySiteAlias')/Plants('MyPlant')/Deliverables('{0001ADB6-0000-0000-9300-1BC0C4591E04}')/Files('0001ADB6-0000-0000-9300-1BC0C4591E04_Default')/$value