Add opening file proxy for windows.

Source/Editor/Content/Proxy/FileProxy.cs

@@ -32,6 +32,14 @@ namespace FlaxEditor.Content
         /// <inheritdoc />
         public override EditorWindow Open(Editor editor, ContentItem item)
         {
+#if PLATFORM_WINDOWS
+            CreateProcessSettings settings = new CreateProcessSettings
+            {
+                ShellExecute = true,
+                FileName = item.Path
+            };
+            Platform.CreateProcess(ref settings);
+#endif
             return null;
         }