I don't think the term "Microkernel Architecture" should be used in this context. I think "Modular Architecture," (or Plug-in like is mentioned) gets closer to this extension-based pattern.
The reason being that there's no relevance to the kernel, and modular kernels, also take this approach with replaceable plug-ins or extensions.
The reason being that there's no relevance to the kernel, and modular kernels, also take this approach with replaceable plug-ins or extensions.