An asset can contain more than one thing which is why that code returns an array but only opens a single thing. Resources.LoadAll works for a folder but only in the resources directory, Resources.FindObjectsOfTypeAll is useful as it will search prefabs but not using a path.
In the editor you can use the usual System.IO functions to search for content in a folder. Now you can filter it manually (by extension / file name / what ever) and use AssetDatabase.LoadAssetAtPath on each asset you want to import.