Allow webp for local images (backport) (#9831)

MediaBrowser.Controller/Entities/BaseItem.cs

@@ -47,7 +47,7 @@ namespace MediaBrowser.Controller.Entities
         /// The supported image extensions.
         /// </summary>
         public static readonly string[] SupportedImageExtensions
-            = new[] { ".png", ".jpg", ".jpeg", ".tbn", ".gif" };
+            = new[] { ".png", ".jpg", ".jpeg", ".webp", ".tbn", ".gif" };
 
         private static readonly List<string> _supportedExtensions = new List<string>(SupportedImageExtensions)
         {