CustomEditor in Files that they dont are ScriptableObject

No, Unity only supports some extensions as TextAssets. Also TextAssets are read-only inside Unity. You can of course use the OS file system functions to "edit" them in the editor, but that's usually not the point of TextAssets. Those are externally created resources which you can simply include and use in Unity.

Though if you use System.IO classes / methods to access / edit the file the extension doesn't matter at all. Just if you want to actually use them in the editor as assets they need to have a supported extension.