Allows setting a string in the Unity Editor by dragging a file or folder from outside the editor.

PathAttribute.cs

using UnityEngine;

public class PathAttribute : PropertyAttribute
{
}

PathAttributeDrawer.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(PathAttribute))]
public class PathAttributeDrawer : PropertyDrawer
{
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        EditorGUI.BeginProperty(position, label, property);
        position = EditorGUI.PrefixLabel(position, GUIUtility.GetControlID(FocusType.Passive), label);
        
        var currentEvent = Event.current;
        var currentEventType = currentEvent.type;
        if (currentEventType == EventType.DragExited) DragAndDrop.PrepareStartDrag();
        if (position.Contains(currentEvent.mousePosition))
        {
            switch (currentEventType)
            {
                case EventType.DragUpdated:
                    DragAndDrop.visualMode = DragAndDropVisualMode.Link;

                    currentEvent.Use();
                    break;

                case EventType.DragPerform:
                    DragAndDrop.AcceptDrag();

                    if (DragAndDrop.paths.Length > 0)
                    {
                        property.stringValue = DragAndDrop.paths[0];

                        currentEvent.Use();
                    }
                    break;
            }
        }

        property.stringValue = EditorGUI.TextField(position, property.stringValue ?? "");

        EditorGUI.EndProperty();
    }
}