use modifiable media path in file system composition

src/Umbraco.Core/Composing/CompositionExtensions/FileSystems.cs

@@ -90,7 +90,7 @@ namespace Umbraco.Core.Composing.CompositionExtensions
             // register the IFileSystem supporting the IMediaFileSystem
             // THIS IS THE ONLY THING THAT NEEDS TO CHANGE, IN ORDER TO REPLACE THE UNDERLYING FILESYSTEM
             // and, SupportingFileSystem.For<IMediaFileSystem>() returns the underlying filesystem
-            composition.SetMediaFileSystem(() => new PhysicalFileSystem("~/media"));
+            composition.SetMediaFileSystem(() => new PhysicalFileSystem(SystemDirectories.Media));
 
             return composition;
         }