How to use DragAndDrop function in Unity3D custom editor. This is just handle when user drag anything outside to THIS EditorWindow only.

DragAndDropEditorWindow.cs

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);
				}
			}
		}
	}
}