プレビューにアセットの情報を表示するにはこのメソッドを使用します。
You will also have to implement HasPreviewGUI and OnPreviewGUI in addition to this method.
The default implementation just returns an empty string, which disables the feature.
Note: Inspector previews are limited to the primary editor of persistent objects (assets). For example, GameObjectInspector, MaterialEditor, TextureInspector, and so on.
This means that it is currently not possible for a component to have its own inspector preview.