How to create a plugin to load custom assets (mesh, animations, etc.)?

I am new to UE4 and I want to load custom file formats by creating an importer plugin.
I’ve searched through previously asked questions but couldn’t quite find what I was hoping for.
I’ve also looked into FBX importer from /Editor/UnrealEd/ but it appears to be a part of the editor instead of a plugin.
Any clues as to how to approach this would be appreciated.