I think the extension arrow type is the 'unfilled' type (i.e. https://www.uml-diagrams.org/association.html?context=class-diagrams) but there are a couple of ways you can have a filled arrow as required:
@startuml
'left to right direction
skinparam shadowing<<stereotype>> false
skinparam shadowing<<metaclass>> false
note "__Extends__ arrow: simple" as notelabel_ab
notelabel_ab .. A
notelabel_ab .. B
class A <<stereotype>>
class B <<metaclass>>
A -|> B: extends
note "__Solid__ arrow: using the label on relations" as notelabel_cd
notelabel_cd .. C
notelabel_cd .. D
class C <<stereotype>>
class D <<metaclass>>
C - D: solid >
note "__Solid__ arrow: original style arrow type" as notelabel_ef
notelabel_ef .. E
notelabel_ef .. F
class E <<stereotype>>
class F <<metaclass>>
E -> F: solid
@enduml