diff --git a/Kyoo/Kyoo.csproj b/Kyoo/Kyoo.csproj
index 205b8458..70fae8ca 100644
--- a/Kyoo/Kyoo.csproj
+++ b/Kyoo/Kyoo.csproj
@@ -32,6 +32,7 @@
 	</PropertyGroup>
 
 	<ItemGroup>
+		<PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
 		<ProjectReference Include="../Kyoo.Common/Kyoo.Common.csproj" />
 		<ProjectReference Include="../Kyoo.CommonAPI/Kyoo.CommonAPI.csproj" />
 		<PackageReference Include="Autofac" Version="6.2.0" />
diff --git a/Kyoo/Models/FileExtensions.cs b/Kyoo/Models/FileExtensions.cs
index b42b4be2..d023c039 100644
--- a/Kyoo/Models/FileExtensions.cs
+++ b/Kyoo/Models/FileExtensions.cs
@@ -1,6 +1,6 @@
 using System.Collections.Generic;
+using System.Collections.Immutable;
 using System.IO;
-using System.Linq;
 
 namespace Kyoo.Models.Watch
 {
@@ -12,7 +12,7 @@ namespace Kyoo.Models.Watch
 		/// <summary>
 		/// The list of known video extensions
 		/// </summary>
-		public static readonly string[] VideoExtensions =
+		public static readonly ImmutableArray<string> VideoExtensions = new()
 		{
 			".webm",
 			".mkv",
@@ -53,11 +53,11 @@ namespace Kyoo.Models.Watch
 		/// <summary>
 		/// The dictionary of known subtitles extensions and the name of the subtitle codec.
 		/// </summary>
-		public static readonly Dictionary<string, string> SubtitleExtensions = new()
+		public static readonly ImmutableDictionary<string, string> SubtitleExtensions =  new Dictionary<string, string> 
 		{
 			{".ass", "ass"},
 			{".str", "subrip"}
-		};
+		}.ToImmutableDictionary();
 
 		/// <summary>
 		/// Check if a file represent a subtitle  file (only by checking the extension of the file)