DragAndDropEditorWindow

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;

public class DragAndDropEditorWindow : EditorWindow
{
    [MenuItem("Window/Drag And Drop")]
    public static void Open()
    {
        GetWindow<DragAndDropEditorWindow>(false, "DragDrop", true);
    }

    private void OnGUI()
    {
        if (Event.current.type == EventType.DragUpdated)
        {
            DragAndDrop.visualMode = DragAndDropVisualMode.Copy;
            Event.current.Use();
        }
        else if (Event.current.type == EventType.DragPerform)
        {
            // To consume drag data.
            DragAndDrop.AcceptDrag();

            // GameObjects from hierarchy.
            if (DragAndDrop.paths.Length == 0 && DragAndDrop.objectReferences.Length > 0)
            {
                Debug.Log("GameObject");
                foreach (Object obj in DragAndDrop.objectReferences)
                {
                    Debug.Log("- " + obj);
                }
            }
            // Object outside project. It mays from File Explorer (Finder in OSX).
            else if (DragAndDrop.paths.Length > 0 && DragAndDrop.objectReferences.Length == 0)
            {
                Debug.Log("File");
                foreach (string path in DragAndDrop.paths)
                {
                    Debug.Log("- " + path);
                }
            }
            // Unity Assets including folder.
            else if (DragAndDrop.paths.Length == DragAndDrop.objectReferences.Length)
            {
                Debug.Log("UnityAsset");
                for (int i = 0; i < DragAndDrop.objectReferences.Length; i++)
                {
                    Object obj = DragAndDrop.objectReferences[i];
                    string path = DragAndDrop.paths[i];
                    Debug.Log(obj.GetType().Name);

                    // Folder.
                    if (obj is DefaultAsset)
                    {
                        Debug.Log(path);
                    }
                    // C# or JavaScript.
                    else if (obj is MonoScript)
                    {
                        Debug.Log(path + "\n" + obj);
                    }
                    else if (obj is Texture2D)
                    {

                    }

                }
            }
            // Log to make sure we cover all cases.
            else
            {
                Debug.Log("Out of reach");
                Debug.Log("Paths:");
                foreach (string path in DragAndDrop.paths)
                {
                    Debug.Log("- " + path);
                }

                Debug.Log("ObjectReferences:");
                foreach (Object obj in DragAndDrop.objectReferences)
                {
                    Debug.Log("- " + obj);
                }
            }
        }
    }
}