Support default extension

src/Avalonia.Base/Platform/Storage/FileIO/StorageProviderHelpers.cs

@@ -50,7 +50,8 @@ internal static class StorageProviderHelpers
         }
     }
     
-    public static string NameWithExtension(string path, string? defaultExtension, FilePickerFileType? filter)
+    [return: NotNullIfNotNull(nameof(path))]
+    public static string? NameWithExtension(string? path, string? defaultExtension, FilePickerFileType? filter)
     {
         var name = Path.GetFileName(path);
         if (name != null && !Path.HasExtension(name))

src/Browser/Avalonia.Browser/Storage/BrowserStorageProvider.cs

@@ -6,6 +6,7 @@ using System.Runtime.InteropServices.JavaScript;
 using System.Threading.Tasks;
 using Avalonia.Browser.Interop;
 using Avalonia.Platform.Storage;
+using Avalonia.Platform.Storage.FileIO;
 
 namespace Avalonia.Browser.Storage;
 
@@ -64,7 +65,9 @@ internal class BrowserStorageProvider : IStorageProvider
 
         try
         {
-            var item = await StorageHelper.SaveFileDialog(startIn, options.SuggestedFileName, types, excludeAll, PreferPolyfill);
+            var suggestedName =
+                StorageProviderHelpers.NameWithExtension(options.SuggestedFileName, options.DefaultExtension, null);
+            var item = await StorageHelper.SaveFileDialog(startIn, suggestedName, types, excludeAll, PreferPolyfill);
             return item is not null ? new JSStorageFile(item) : null;
         }
         catch (JSException ex) when (ex.Message.Contains(PickerCancelMessage, StringComparison.Ordinal))