diff --git a/OpenRA.Game/FileSystem/FileSystem.cs b/OpenRA.Game/FileSystem/FileSystem.cs
index 018393a19c..c59371e8c4 100644
--- a/OpenRA.Game/FileSystem/FileSystem.cs
+++ b/OpenRA.Game/FileSystem/FileSystem.cs
@@ -17,7 +17,15 @@ using OpenRA.Primitives;
 
 namespace OpenRA.FileSystem
 {
-	public class FileSystem
+	public interface IReadOnlyFileSystem
+	{
+		Stream Open(string filename);
+		bool TryGetPackageContaining(string path, out IReadOnlyPackage package, out string filename);
+		bool TryOpen(string filename, out Stream s);
+		bool Exists(string filename);
+	}
+
+	public class FileSystem : IReadOnlyFileSystem
 	{
 		public IEnumerable<IReadOnlyPackage> MountedPackages { get { return mountedPackages.Keys; } }
 		readonly Dictionary<IReadOnlyPackage, int> mountedPackages = new Dictionary<IReadOnlyPackage, int>();
diff --git a/OpenRA.Game/ModData.cs b/OpenRA.Game/ModData.cs
index f4523460b7..a5ff4915c1 100644
--- a/OpenRA.Game/ModData.cs
+++ b/OpenRA.Game/ModData.cs
@@ -13,6 +13,7 @@ using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
+using OpenRA.FileSystem;
 using OpenRA.Graphics;
 using OpenRA.Widgets;
 using FS = OpenRA.FileSystem.FileSystem;
@@ -36,6 +37,7 @@ namespace OpenRA
 
 		readonly Lazy<Ruleset> defaultRules;
 		public Ruleset DefaultRules { get { return defaultRules.Value; } }
+		public IReadOnlyFileSystem DefaultFileSystem { get { return ModFiles; } }
 
 		public ModData(string mod, bool useLoadScreen = false)
 		{