The editor is using Assimpjs (https://github.com/kovacsv/assimpjs) to load 3d models that are not .gltf and not .babylon.
The editor provides a SceneLoader plugin that uses Assimpjs.

Bones are still not supported for some 3d models such as the ones coming from mixamo (https://www.mixamo.com/). The editor should be able to import any FBX with full bones support.