diff --git a/OpenRA.Game/FileSystem/BagFile.cs b/OpenRA.Game/FileSystem/BagFile.cs
index 67ee2fdfcf..2c24d232ac 100644
--- a/OpenRA.Game/FileSystem/BagFile.cs
+++ b/OpenRA.Game/FileSystem/BagFile.cs
@@ -19,7 +19,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class BagFile : IPackage
+	public sealed class BagFile : IReadOnlyPackage
 	{
 		static readonly uint[] Nothing = { };
 
@@ -175,12 +175,6 @@ namespace OpenRA.FileSystem
 			return index.Keys.Select(k => lookup.ContainsKey(k) ? lookup[k] : "{0:X}".F(k));
 		}
 
-		public void Write(Dictionary<string, byte[]> contents)
-		{
-			context.Unmount(this);
-			throw new NotImplementedException("Updating bag files unsupported");
-		}
-
 		public void Dispose()
 		{
 			s.Dispose();
diff --git a/OpenRA.Game/FileSystem/BigFile.cs b/OpenRA.Game/FileSystem/BigFile.cs
index ec3dc977a4..8d8dc559ef 100644
--- a/OpenRA.Game/FileSystem/BigFile.cs
+++ b/OpenRA.Game/FileSystem/BigFile.cs
@@ -15,7 +15,7 @@ using System.Linq;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class BigFile : IPackage
+	public sealed class BigFile : IReadOnlyPackage
 	{
 		public string Name { get; private set; }
 		public int Priority { get; private set; }
@@ -111,11 +111,6 @@ namespace OpenRA.FileSystem
 			return entries.Keys;
 		}
 
-		public void Write(Dictionary<string, byte[]> contents)
-		{
-			throw new NotImplementedException();
-		}
-
 		public void Dispose()
 		{
 			s.Dispose();
diff --git a/OpenRA.Game/FileSystem/D2kSoundResources.cs b/OpenRA.Game/FileSystem/D2kSoundResources.cs
index 852a5160b2..2e2bc35c96 100644
--- a/OpenRA.Game/FileSystem/D2kSoundResources.cs
+++ b/OpenRA.Game/FileSystem/D2kSoundResources.cs
@@ -14,7 +14,7 @@ using System.IO;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class D2kSoundResources : IPackage
+	public sealed class D2kSoundResources : IReadOnlyPackage
 	{
 		readonly Stream s;
 
@@ -94,11 +94,6 @@ namespace OpenRA.FileSystem
 			yield break;
 		}
 
-		public void Write(Dictionary<string, byte[]> contents)
-		{
-			throw new NotImplementedException("Cannot save Dune 2000 Sound Resources.");
-		}
-
 		public void Dispose()
 		{
 			s.Dispose();
diff --git a/OpenRA.Game/FileSystem/FileSystem.cs b/OpenRA.Game/FileSystem/FileSystem.cs
index 1235d43ae8..3658ab36d2 100644
--- a/OpenRA.Game/FileSystem/FileSystem.cs
+++ b/OpenRA.Game/FileSystem/FileSystem.cs
@@ -20,37 +20,25 @@ namespace OpenRA.FileSystem
 	public class FileSystem
 	{
 		public readonly List<string> PackagePaths = new List<string>();
-		public readonly List<IPackage> MountedPackages = new List<IPackage>();
+		public readonly List<IReadOnlyPackage> MountedPackages = new List<IReadOnlyPackage>();
 
 		static readonly Dictionary<string, Assembly> AssemblyCache = new Dictionary<string, Assembly>();
 
 		int order;
-		Cache<uint, List<IPackage>> crcHashIndex = new Cache<uint, List<IPackage>>(_ => new List<IPackage>());
-		Cache<uint, List<IPackage>> classicHashIndex = new Cache<uint, List<IPackage>>(_ => new List<IPackage>());
+		Cache<uint, List<IReadOnlyPackage>> crcHashIndex = new Cache<uint, List<IReadOnlyPackage>>(_ => new List<IReadOnlyPackage>());
+		Cache<uint, List<IReadOnlyPackage>> classicHashIndex = new Cache<uint, List<IReadOnlyPackage>>(_ => new List<IReadOnlyPackage>());
 
-		public IPackage CreatePackage(string filename, int order, Dictionary<string, byte[]> content)
+		public IReadWritePackage CreatePackage(string filename, int order, Dictionary<string, byte[]> content)
 		{
-			if (filename.EndsWith(".mix", StringComparison.InvariantCultureIgnoreCase))
-				return new MixFile(this, filename, order, content);
 			if (filename.EndsWith(".zip", StringComparison.InvariantCultureIgnoreCase))
 				return new ZipFile(this, filename, order, content);
 			if (filename.EndsWith(".oramap", StringComparison.InvariantCultureIgnoreCase))
 				return new ZipFile(this, filename, order, content);
-			if (filename.EndsWith(".RS", StringComparison.InvariantCultureIgnoreCase))
-				throw new NotImplementedException("The creation of .RS archives is unimplemented");
-			if (filename.EndsWith(".Z", StringComparison.InvariantCultureIgnoreCase))
-				throw new NotImplementedException("The creation of .Z archives is unimplemented");
-			if (filename.EndsWith(".PAK", StringComparison.InvariantCultureIgnoreCase))
-				throw new NotImplementedException("The creation of .PAK archives is unimplemented");
-			if (filename.EndsWith(".big", StringComparison.InvariantCultureIgnoreCase))
-				throw new NotImplementedException("The creation of .big archives is unimplemented");
-			if (filename.EndsWith(".cab", StringComparison.InvariantCultureIgnoreCase))
-				throw new NotImplementedException("The creation of .cab archives is unimplemented");
 
 			return new Folder(filename, order, content);
 		}
 
-		public IPackage OpenPackage(string filename, string annotation, int order)
+		public IReadOnlyPackage OpenPackage(string filename, string annotation, int order)
 		{
 			if (filename.EndsWith(".mix", StringComparison.InvariantCultureIgnoreCase))
 			{
@@ -81,7 +69,17 @@ namespace OpenRA.FileSystem
 			return new Folder(filename, order);
 		}
 
-		public void Mount(IPackage mount)
+		public IReadWritePackage OpenWritablePackage(string filename, int order)
+		{
+			if (filename.EndsWith(".zip", StringComparison.InvariantCultureIgnoreCase))
+				return new ZipFile(this, filename, order);
+			if (filename.EndsWith(".oramap", StringComparison.InvariantCultureIgnoreCase))
+				return new ZipFile(this, filename, order);
+
+			return new Folder(filename, order);
+		}
+
+		public void Mount(IReadOnlyPackage mount)
 		{
 			if (!MountedPackages.Contains(mount))
 				MountedPackages.Add(mount);
@@ -105,7 +103,7 @@ namespace OpenRA.FileSystem
 				a();
 		}
 
-		void MountInner(IPackage package)
+		void MountInner(IReadOnlyPackage package)
 		{
 			MountedPackages.Add(package);
 
@@ -124,7 +122,7 @@ namespace OpenRA.FileSystem
 			}
 		}
 
-		public bool Unmount(IPackage mount)
+		public bool Unmount(IReadOnlyPackage mount)
 		{
 			if (MountedPackages.Contains(mount))
 				mount.Dispose();
@@ -139,8 +137,8 @@ namespace OpenRA.FileSystem
 
 			MountedPackages.Clear();
 			PackagePaths.Clear();
-			classicHashIndex = new Cache<uint, List<IPackage>>(_ => new List<IPackage>());
-			crcHashIndex = new Cache<uint, List<IPackage>>(_ => new List<IPackage>());
+			classicHashIndex = new Cache<uint, List<IReadOnlyPackage>>(_ => new List<IReadOnlyPackage>());
+			crcHashIndex = new Cache<uint, List<IReadOnlyPackage>>(_ => new List<IReadOnlyPackage>());
 		}
 
 		public void LoadFromManifest(Manifest manifest)
@@ -203,7 +201,7 @@ namespace OpenRA.FileSystem
 			}
 
 			// Ask each package individually
-			IPackage package;
+			IReadOnlyPackage package;
 			if (explicitPackage && !string.IsNullOrEmpty(packageName))
 				package = MountedPackages.Where(x => x.Name == packageName).MaxByOrDefault(x => x.Priority);
 			else
diff --git a/OpenRA.Game/FileSystem/Folder.cs b/OpenRA.Game/FileSystem/Folder.cs
index 920948b22e..5fe419b2f9 100644
--- a/OpenRA.Game/FileSystem/Folder.cs
+++ b/OpenRA.Game/FileSystem/Folder.cs
@@ -13,7 +13,7 @@ using System.IO;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class Folder : IPackage
+	public sealed class Folder : IReadWritePackage
 	{
 		readonly string path;
 		readonly int priority;
diff --git a/OpenRA.Game/FileSystem/IPackage.cs b/OpenRA.Game/FileSystem/IPackage.cs
index 226ba0bd62..e336b0602e 100644
--- a/OpenRA.Game/FileSystem/IPackage.cs
+++ b/OpenRA.Game/FileSystem/IPackage.cs
@@ -14,15 +14,19 @@ using System.IO;
 
 namespace OpenRA.FileSystem
 {
-	public interface IPackage : IDisposable
+	public interface IReadOnlyPackage : IDisposable
 	{
 		Stream GetContent(string filename);
 		bool Exists(string filename);
 		IEnumerable<uint> ClassicHashes();
 		IEnumerable<uint> CrcHashes();
 		IEnumerable<string> AllFileNames();
-		void Write(Dictionary<string, byte[]> contents);
 		int Priority { get; }
 		string Name { get; }
 	}
+
+	public interface IReadWritePackage : IReadOnlyPackage
+	{
+		void Write(Dictionary<string, byte[]> contents);
+	}
 }
diff --git a/OpenRA.Game/FileSystem/InstallShieldCABExtractor.cs b/OpenRA.Game/FileSystem/InstallShieldCABExtractor.cs
index f4e8e22de0..ac9deb296c 100644
--- a/OpenRA.Game/FileSystem/InstallShieldCABExtractor.cs
+++ b/OpenRA.Game/FileSystem/InstallShieldCABExtractor.cs
@@ -17,7 +17,7 @@ using ICSharpCode.SharpZipLib.Zip.Compression;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class InstallShieldCABExtractor : IPackage
+	public sealed class InstallShieldCABExtractor : IReadOnlyPackage
 	{
 		const uint FileSplit = 0x1;
 		const uint FileObfuscated = 0x2;
@@ -451,11 +451,6 @@ namespace OpenRA.FileSystem
 				GetContentById(index, destfile);
 		}
 
-		public void Write(Dictionary<string, byte[]> input)
-		{
-			throw new NotImplementedException("Cannot Add Files To Cab");
-		}
-
 		public IEnumerable<uint> ClassicHashes()
 		{
 			return fileLookup.Keys.Select(k => PackageEntry.HashFilename(k, PackageHashType.Classic));
diff --git a/OpenRA.Game/FileSystem/InstallShieldPackage.cs b/OpenRA.Game/FileSystem/InstallShieldPackage.cs
index 2355209ff4..dd28d1042d 100644
--- a/OpenRA.Game/FileSystem/InstallShieldPackage.cs
+++ b/OpenRA.Game/FileSystem/InstallShieldPackage.cs
@@ -15,7 +15,7 @@ using OpenRA.FileFormats;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class InstallShieldPackage : IPackage
+	public sealed class InstallShieldPackage : IReadOnlyPackage
 	{
 		readonly Dictionary<uint, PackageEntry> index = new Dictionary<uint, PackageEntry>();
 		readonly List<string> filenames;
diff --git a/OpenRA.Game/FileSystem/MixFile.cs b/OpenRA.Game/FileSystem/MixFile.cs
index 0faaeb0c55..25532a8ff5 100644
--- a/OpenRA.Game/FileSystem/MixFile.cs
+++ b/OpenRA.Game/FileSystem/MixFile.cs
@@ -18,7 +18,7 @@ using OpenRA.Primitives;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class MixFile : IPackage
+	public sealed class MixFile : IReadOnlyPackage
 	{
 		readonly Dictionary<uint, PackageEntry> index;
 		readonly long dataStart;
@@ -28,31 +28,6 @@ namespace OpenRA.FileSystem
 		readonly FileSystem context;
 		readonly PackageHashType type;
 
-		// Save a mix to disk with the given contents
-		public MixFile(FileSystem context, string filename, int priority, Dictionary<string, byte[]> contents)
-		{
-			this.filename = filename;
-			this.priority = priority;
-			this.type = PackageHashType.Classic;
-			this.context = context;
-
-			if (File.Exists(filename))
-				File.Delete(filename);
-
-			s = File.Create(filename);
-			try
-			{
-				index = new Dictionary<uint, PackageEntry>();
-				contents.Add("local mix database.dat", new XccLocalDatabase(contents.Keys.Append("local mix database.dat")).Data());
-				Write(contents);
-			}
-			catch
-			{
-				Dispose();
-				throw;
-			}
-		}
-
 		public MixFile(FileSystem context, string filename, PackageHashType type, int priority)
 		{
 			this.filename = filename;
@@ -248,45 +223,6 @@ namespace OpenRA.FileSystem
 		public int Priority { get { return 1000 + priority; } }
 		public string Name { get { return filename; } }
 
-		public void Write(Dictionary<string, byte[]> contents)
-		{
-			// Cannot modify existing mixfile - rename existing file and
-			// create a new one with original content plus modifications
-			context.Unmount(this);
-
-			// TODO: Add existing data to the contents list
-			if (index.Count > 0)
-				throw new NotImplementedException("Updating mix files unfinished");
-
-			// Construct a list of entries for the file header
-			uint dataSize = 0;
-			var items = new List<PackageEntry>();
-			foreach (var kv in contents)
-			{
-				var length = (uint)kv.Value.Length;
-				var hash = PackageEntry.HashFilename(Path.GetFileName(kv.Key), type);
-				items.Add(new PackageEntry(hash, dataSize, length));
-				dataSize += length;
-			}
-
-			// Write the new file
-			s.Seek(0, SeekOrigin.Begin);
-			using (var writer = new BinaryWriter(s))
-			{
-				// Write file header
-				writer.Write((ushort)items.Count);
-				writer.Write(dataSize);
-				foreach (var item in items)
-					item.Write(writer);
-
-				writer.Flush();
-
-				// Copy file data
-				foreach (var file in contents)
-					s.Write(file.Value);
-			}
-		}
-
 		public void Dispose()
 		{
 			s.Dispose();
diff --git a/OpenRA.Game/FileSystem/Pak.cs b/OpenRA.Game/FileSystem/Pak.cs
index d3c0d7e764..1e9f8727a2 100644
--- a/OpenRA.Game/FileSystem/Pak.cs
+++ b/OpenRA.Game/FileSystem/Pak.cs
@@ -21,7 +21,7 @@ namespace OpenRA.FileSystem
 		public string Filename;
 	}
 
-	public sealed class PakFile : IPackage
+	public sealed class PakFile : IReadOnlyPackage
 	{
 		readonly string filename;
 		readonly int priority;
@@ -93,11 +93,6 @@ namespace OpenRA.FileSystem
 			return index.ContainsKey(filename);
 		}
 
-		public void Write(Dictionary<string, byte[]> contents)
-		{
-			throw new NotImplementedException("Cannot save Pak archives.");
-		}
-
 		public int Priority { get { return 1000 + priority; } }
 		public string Name { get { return filename; } }
 
diff --git a/OpenRA.Game/FileSystem/ZipFile.cs b/OpenRA.Game/FileSystem/ZipFile.cs
index 9699b0b216..bc8bb639cd 100644
--- a/OpenRA.Game/FileSystem/ZipFile.cs
+++ b/OpenRA.Game/FileSystem/ZipFile.cs
@@ -17,7 +17,7 @@ using SZipFile = ICSharpCode.SharpZipLib.Zip.ZipFile;
 
 namespace OpenRA.FileSystem
 {
-	public sealed class ZipFile : IPackage
+	public sealed class ZipFile : IReadWritePackage
 	{
 		readonly string filename;
 		readonly int priority;
diff --git a/OpenRA.Game/Map/Map.cs b/OpenRA.Game/Map/Map.cs
index c7060c8e6b..84c0db3cc4 100644
--- a/OpenRA.Game/Map/Map.cs
+++ b/OpenRA.Game/Map/Map.cs
@@ -157,7 +157,7 @@ namespace OpenRA
 		[FieldLoader.Ignore] public readonly WVec[] SubCellOffsets;
 		public readonly SubCell DefaultSubCell;
 		public readonly SubCell LastSubCell;
-		[FieldLoader.Ignore] public IPackage Container;
+		[FieldLoader.Ignore] public IReadWritePackage Container;
 		public string Path { get; private set; }
 
 		// Yaml map data
@@ -317,7 +317,7 @@ namespace OpenRA
 		public Map(string path)
 		{
 			Path = path;
-			Container = Game.ModData.ModFiles.OpenPackage(path, null, int.MaxValue);
+			Container = Game.ModData.ModFiles.OpenWritablePackage(path, int.MaxValue);
 
 			AssertExists("map.yaml");
 			AssertExists("map.bin");
diff --git a/OpenRA.Mods.Common/Widgets/Logic/AssetBrowserLogic.cs b/OpenRA.Mods.Common/Widgets/Logic/AssetBrowserLogic.cs
index b878bf343c..8de9d08930 100644
--- a/OpenRA.Mods.Common/Widgets/Logic/AssetBrowserLogic.cs
+++ b/OpenRA.Mods.Common/Widgets/Logic/AssetBrowserLogic.cs
@@ -33,7 +33,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 		ScrollPanelWidget assetList;
 		ScrollItemWidget template;
 
-		IPackage assetSource = null;
+		IReadOnlyPackage assetSource = null;
 		List<string> availableShps = new List<string>();
 		bool animateFrames = false;
 
@@ -334,7 +334,7 @@ namespace OpenRA.Mods.Common.Widgets.Logic
 
 		bool ShowSourceDropdown(DropDownButtonWidget dropdown)
 		{
-			Func<IPackage, ScrollItemWidget, ScrollItemWidget> setupItem = (source, itemTemplate) =>
+			Func<IReadOnlyPackage, ScrollItemWidget, ScrollItemWidget> setupItem = (source, itemTemplate) =>
 			{
 				var item = ScrollItemWidget.Setup(itemTemplate,
 					() => assetSource == source,