[CustomPropertyDrawer(typeof(TestSprite))]
public class TestSpriteDrawer : PropertyDrawer
{
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        float spriteHeight = 130f;
        float restHeight = EditorGUI.GetPropertyHeight(property, label, true);
        return spriteHeight + restHeight;
    }

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        EditorGUI.BeginProperty(position, label, property);

        SerializedProperty spriteProp = property.FindPropertyRelative("mySpriteTest");

        Rect spriteRect = new Rect(position.x, position.y, 128, 128);
        Object newSprite = EditorGUI.ObjectField(spriteRect, spriteProp.objectReferenceValue, typeof(Sprite), false);
        if (newSprite != spriteProp.objectReferenceValue)
        {
            spriteProp.objectReferenceValue = newSprite;
        }

        Sprite sprite = spriteProp.objectReferenceValue as Sprite;
        if (sprite != null)
        {
            Texture2D texture = AssetPreview.GetAssetPreview(sprite);

            if (texture != null) GUI.DrawTexture(spriteRect, texture, ScaleMode.ScaleToFit);
        }

        float offsetY = spriteRect.yMax + 4f;
        Rect restRect = new Rect(position.x, offsetY, position.width, position.height - offsetY);

        EditorGUI.PropertyField(restRect, property, label, true);

        EditorGUI.EndProperty();
    }
}