The CopyFile extension method was not disposing the file stream, so any attempt to delete the source file afterwards would fail.

Removing unused using statements.

src/Umbraco.Core/IO/FileSystemExtensions.cs

@@ -1,6 +1,5 @@
 using System;
 using System.IO;
-using Umbraco.Core.CodeAnnotations;
 
 namespace Umbraco.Core.IO
 {	
@@ -19,7 +18,10 @@ namespace Umbraco.Core.IO
 
         public static void CopyFile(this IFileSystem fs, string path, string newPath)
         {
-            fs.AddFile(newPath, fs.OpenFile(path));
+            using (var fileStream = fs.OpenFile(path))
+            {
+                fs.AddFile(newPath, fileStream);
+            }
         }
 
         public static string GetExtension(this IFileSystem fs, string path)

src/Umbraco.Core/IO/FileSystemWrapper.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
-using Umbraco.Core.CodeAnnotations;
 
 namespace Umbraco.Core.IO
 {

src/Umbraco.Core/IO/IFileSystem.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
-using Umbraco.Core.CodeAnnotations;
 
 namespace Umbraco.Core.IO
 {

src/Umbraco.Core/IO/PhysicalFileSystem.cs

@@ -3,10 +3,7 @@ using System.Collections.Generic;
 using System.Globalization;
 using System.IO; 
 using System.Linq;
-using System.Web;
-using Umbraco.Core.CodeAnnotations;
 using Umbraco.Core.Logging;
-using Umbraco.Core.Publishing;
 
 namespace Umbraco.Core.IO
 {