mod:register({
    DataType = "BUILDING_PART",
    ...

--  OLD
--  BuildingFunction = { 
--      DataType = "BUILDING_FUNCTION_WORKPLACE",
--      WorkerCapacity = 4,
--      RelatedJob = { Job = "MITHRIL_MINER", Behavior = "WORK_BEHAVIOR" },
--      ResourceProduced = {
--          { Resource = "MITHRIL_ORE", Quantity = 5 }
--      }
--  },

--  NEW
    BuildingFunction = "MITHRIL_MINE_FUNCTION",

    ...
})

-- ALSO NEW
mod:register({
    DataType = "BUILDING_FUNCTION_WORKPLACE",
    Id = "MITHRIL_MINE_FUNCTION",
    WorkerCapacity = 4,
    RelatedJob = { Job = "MITHRIL_MINER", Behavior = "WORK_BEHAVIOR" },
    ResourceProduced = {
        { Resource = "MITHRIL_ORE", Quantity = 5 }
    }
})