尝试在.NET MAUI应用的项目清单中声明文件类型扩展名和文件类型关联。打开MauiProject.csproj文件并在元素中添加以下代码：

<ItemGroup>
  <AppExtension Include=".YOURCUSTOMFILEEXTENSION">
    <ContentType>Text</ContentType>
    <RequestAccess>true</RequestAccess>
  </AppExtension>
</ItemGroup>

然后尝试在MauiProgram.cs文件中添加文件激活事件处理程序。

public static MauiApp CreateMauiApp()
        {
            var builder = MauiApp.CreateBuilder();
            builder
                .ConfigureLifecycleEvents(lifecycle =>
                {
                    lifecycle.AddWindows<FileActivatedHandler>((_, args) =>
                    {
                        // Handle the file activation event
                        if (args.Files?.Count > 0)
                        {
                            var file = args.Files[0];
                            var filePath = file.Path;
                            // Process the file as needed
                            // For example, open the file in your app's editor
                            OpenFile(filePath);
                        }
                    });
                })
                .ConfigureMauiHandlers(handlers =>
                {
                    // Register your app's handlers here
                })
                .UseMauiApp<App>()
                .ConfigureFonts(fonts =>
                {
                    fonts.AddFont("OpenSans-Regular.ttf", "OpenSansRegular");
                });

            return builder.Build();
        }

        private static void OpenFile(string filePath)
        {
            // Open the file in your app's editor
            // You can use the filePath to access the file contents and work with it
            // For example:
            // var content = File.ReadAllText(filePath);
            // Perform any necessary operations on the file content

            //await Launcher.OpenAsync(new OpenFileRequest
            //{
            //    File = new ReadOnlyFile(filePath)
            //});
        }