<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        Height="300" Width="300"
        x:Class="AvaloniaUI.OpenFileTest"
        Title="OpenFileTest">
    <DockPanel Margin="5">
        <Button DockPanel.Dock="Top" Click="cmdOpen_Click" HorizontalAlignment="Center">Open</Button>
        <ListBox Name="lstFiles"></ListBox>
    </DockPanel>
</Window>

using Avalonia;
using Avalonia.Controls;
using Avalonia.Interactivity;
using Avalonia.Markup.Xaml;
using Avalonia.Platform.Storage;
using System.Linq;

namespace AvaloniaUI;

public partial class OpenFileTest : Window
{
    public OpenFileTest()
    {
        InitializeComponent();
    }

    private async void cmdOpen_Click(object? sender, RoutedEventArgs e)
    {
        var options = new FilePickerOpenOptions
        {
            Title = "Open",
            AllowMultiple = true,
            FileTypeFilter =
            [
              new FilePickerFileType("Image Files")
                {
                    Patterns = [ "*.bmp", "*.jpg", "*.jpeg", "*.gif" ]
                },
                FilePickerFileTypes.All
            ]
        };

        var files = await StorageProvider.OpenFilePickerAsync(options);
        if (files == null || files.Count == 0)
            return;

        lstFiles.ItemsSource = files.Select(f => f.TryGetLocalPath() ?? f.Name).ToArray();
    }
}