Fixes U4-1689 and U4-1719.

Refactoring UmbracoImageMediaFactory so it uses the extension method for uploading/setting images, so its a bit more DRY.

src/Umbraco.Core/IO/MediaFileSystem.cs

@@ -1,8 +1,6 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
-using System.Text;
-using Umbraco.Core.CodeAnnotations;
 using Umbraco.Core.Configuration;
 
 namespace Umbraco.Core.IO
@@ -27,6 +25,15 @@ namespace Umbraco.Core.IO
 			return propertyId.ToString() + seperator + fileName;
 		}
 
+        public string GetRelativePath(string subfolder, string fileName)
+        {
+            var seperator = UmbracoSettings.UploadAllowDirectories
+                ? Path.DirectorySeparatorChar
+                : '-';
+
+            return subfolder + seperator + fileName;
+        }
+
 		public IEnumerable<string> GetThumbnails(string path)
 		{
 			var parentDirectory = System.IO.Path.GetDirectoryName(path);