UWP file access

I am using Unity 5.3.1, VS 2015 on a Windows 10 machine.
When I am trying to read the contents of a directory I use the following code:

Code (CSharp):

1.     public IEnumerator LoadImages(string Album)
2.     {
3.         WWW w;
4.         Texture2D t2D;
5.  
6.         //WORKING
7.         //string fn = @"D:\Daten\Benutzerprofile\Christian\Pictures\DSC00044.JPG";
8.         //NOT WORKING, BECAUSE Environment... not available
9.         foreach (string fn in Directory.GetFiles(Environment.GetFolderPath(Environment.SpecialFolder.MyPictures) + "\\" + Album, "*.jpg"))
10.         {
11.             w = new WWW("file://" + fn.Replace("\\", "/"));
12.             yield return w;
13.             if (w.error != null)
14.             {
15.                 Debug.Log("Error: " + w.error);
16.                 yield break;
17.             }
18.             else
19.             {
20.                 t2D = new Texture2D(256, 256, TextureFormat.DXT1, true);
21.                 w.LoadImageIntoTexture(t2D);
22.                 Images.Add(t2D);
23.             }
24.         }
25.     }

The code works fine within the editor (on my windows 10 machine!), but when I build the game I end up with an compile error "Environment contains no definition for GetFolderPath" (same with SpecialFolder) in the built uwp solution.

If I only access ONE file (see code above //string fn = "D:\Daten\Benutzerprofile\Christian\Pictures\DSC00044.JPG" it works in the built uwp version too, so the only problem is to get all files of a (well known) directory ...

What is the recommend way for accessing local files (when building a cross platform game with unity)?