Add PreferFileDialogPolyfill option

src/Browser/Avalonia.Browser/BrowserAppBuilder.cs

@@ -11,6 +11,13 @@ public class BrowserPlatformOptions
     /// If null, default path resolved depending on the backend (browser or blazor) is used.
     /// </summary>
     public Func<string, string>? FrameworkAssetPathResolver { get; set; }
+    
+    /// <summary>
+    /// Avalonia uses "native-file-system-adapter" polyfill for the file dialogs.
+    /// If native implementation is available, by default it is used.
+    /// This property forces polyfill to be always used.
+    /// </summary>
+    public bool PreferFileDialogPolyfill { get; set; }
 }
 
 public static class BrowserAppBuilder

src/Browser/Avalonia.Browser/Interop/StorageHelper.cs

@@ -9,15 +9,15 @@ internal static partial class StorageHelper
     public static partial bool HasNativeFilePicker();
 
     [JSImport("StorageProvider.selectFolderDialog", AvaloniaModule.StorageModuleName)]
-    public static partial Task<JSObject?> SelectFolderDialog(JSObject? startIn);
+    public static partial Task<JSObject?> SelectFolderDialog(JSObject? startIn, bool preferPolyfill);
 
     [JSImport("StorageProvider.openFileDialog", AvaloniaModule.StorageModuleName)]
     public static partial Task<JSObject?> OpenFileDialog(JSObject? startIn, bool multiple,
-        [JSMarshalAs<JSType.Array<JSType.Any>>] object[]? types, bool excludeAcceptAllOption);
+        [JSMarshalAs<JSType.Array<JSType.Any>>] object[]? types, bool excludeAcceptAllOption, bool preferPolyfill);
 
     [JSImport("StorageProvider.saveFileDialog", AvaloniaModule.StorageModuleName)]
     public static partial Task<JSObject?> SaveFileDialog(JSObject? startIn, string? suggestedName,
-        [JSMarshalAs<JSType.Array<JSType.Any>>] object[]? types, bool excludeAcceptAllOption);
+        [JSMarshalAs<JSType.Array<JSType.Any>>] object[]? types, bool excludeAcceptAllOption, bool preferPolyfill);
 
     [JSImport("StorageItem.createWellKnownDirectory", AvaloniaModule.StorageModuleName)]
     public static partial JSObject CreateWellKnownDirectory(string wellKnownDirectory);

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

@@ -20,6 +20,9 @@ internal class BrowserStorageProvider : IStorageProvider
     public bool CanSave => StorageHelper.HasNativeFilePicker();
     public bool CanPickFolder => true;
 
+    private bool PreferPolyfill =>
+        AvaloniaLocator.Current.GetService<BrowserPlatformOptions>()?.PreferFileDialogPolyfill ?? false;
+    
     public async Task<IReadOnlyList<IStorageFile>> OpenFilePickerAsync(FilePickerOpenOptions options)
     {
         await AvaloniaModule.ImportStorage();
@@ -29,7 +32,7 @@ internal class BrowserStorageProvider : IStorageProvider
 
         try
         {
-            using var items = await StorageHelper.OpenFileDialog(startIn, options.AllowMultiple, types, excludeAll);
+            using var items = await StorageHelper.OpenFileDialog(startIn, options.AllowMultiple, types, excludeAll, PreferPolyfill);
             if (items is null)
             {
                 return Array.Empty<IStorageFile>();
@@ -63,7 +66,7 @@ internal class BrowserStorageProvider : IStorageProvider
 
         try
         {
-            var item = await StorageHelper.SaveFileDialog(startIn, options.SuggestedFileName, types, excludeAll);
+            var item = await StorageHelper.SaveFileDialog(startIn, options.SuggestedFileName, types, excludeAll, PreferPolyfill);
             return item is not null ? new JSStorageFile(item) : null;
         }
         catch (JSException ex) when (ex.Message.Contains(PickerCancelMessage, StringComparison.Ordinal))
@@ -89,7 +92,7 @@ internal class BrowserStorageProvider : IStorageProvider
 
         try
         {
-            var item = await StorageHelper.SelectFolderDialog(startIn);
+            var item = await StorageHelper.SelectFolderDialog(startIn, PreferPolyfill);
             return item is not null ? new[] { new JSStorageFolder(item) } : Array.Empty<IStorageFolder>();
         }
         catch (JSException ex) when (ex.Message.Contains(PickerCancelMessage, StringComparison.Ordinal))

src/Browser/Avalonia.Browser/webapp/modules/storage/storageProvider.ts

@@ -12,10 +12,12 @@ declare global {
 
 export class StorageProvider {
     public static async selectFolderDialog(
-        startIn: StorageItem | null): Promise<StorageItem> {
+        startIn: StorageItem | null,
+        preferPolyfill: boolean): Promise<StorageItem> {
         // 'Picker' API doesn't accept "null" as a parameter, so it should be set to undefined.
         const options = {
-            startIn: (startIn?.wellKnownType ?? startIn?.handle ?? undefined)
+            startIn: (startIn?.wellKnownType ?? startIn?.handle ?? undefined),
+            _preferPolyfill: preferPolyfill
         };
 
         const handle = await showDirectoryPicker(options as any);
@@ -24,12 +26,14 @@ export class StorageProvider {
 
     public static async openFileDialog(
         startIn: StorageItem | null, multiple: boolean,
-        types: FilePickerAcceptType[] | null, excludeAcceptAllOption: boolean): Promise<StorageItems> {
+        types: FilePickerAcceptType[] | null, excludeAcceptAllOption: boolean,
+        preferPolyfill: boolean): Promise<StorageItems> {
         const options = {
             startIn: (startIn?.wellKnownType ?? startIn?.handle ?? undefined),
             multiple,
             excludeAcceptAllOption,
-            types: (types ?? undefined)
+            types: (types ?? undefined),
+            _preferPolyfill: preferPolyfill
         };
 
         const handles = await showOpenFilePicker(options);
@@ -38,12 +42,14 @@ export class StorageProvider {
 
     public static async saveFileDialog(
         startIn: StorageItem | null, suggestedName: string | null,
-        types: FilePickerAcceptType[] | null, excludeAcceptAllOption: boolean): Promise<StorageItem> {
+        types: FilePickerAcceptType[] | null, excludeAcceptAllOption: boolean,
+        preferPolyfill: boolean): Promise<StorageItem> {
         const options = {
             startIn: (startIn?.wellKnownType ?? startIn?.handle ?? undefined),
             suggestedName: (suggestedName ?? undefined),
             excludeAcceptAllOption,
-            types: (types ?? undefined)
+            types: (types ?? undefined),
+            _preferPolyfill: preferPolyfill
         };
 
         const handle = await showSaveFilePicker(options);