Texblock destinado a receber o nome de um arquivo que limita a quantidade de caracteres mas sempre mostra a extensão.

MainWindow.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Selecionar" HorizontalAlignment="Left" VerticalAlignment="Top" Width="75" Margin="274,10,0,0" Click="Button_Click_1"/>
        <Border HorizontalAlignment="Left" VerticalAlignment="Top" Margin="10,10,0,0" Width="259" BorderBrush="Black" BorderThickness="1" >
            <TextBlock x:Name="TxBArquivo" Text="Nenhum arquivo selecionado" Height="16"/>
        </Border>
    </Grid>
</Window>

MainWindow.xaml.cs

using System.Globalization;
using System.Text;
using System.Text.RegularExpressions;
using System.Windows;
using System.Windows.Media;
using Microsoft.Win32;

namespace WpfApplication1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            var dlg = new OpenFileDialog();
            dlg.FileName = "Document";
            var showDialog = dlg.ShowDialog();
            if (showDialog.HasValue)
            {
                var passouDoTamanho = false;
                var extension = Regex.Match(dlg.FileName, @"[.]{1}(?<extension>[\w]+)$").Groups["extension"].Value;

                var extensionSize = MeasureTextSize(extension, TxBArquivo.FontFamily, TxBArquivo.FontStyle, TxBArquivo.FontWeight, TxBArquivo.FontStretch, TxBArquivo.FontSize);
                var dotSize = MeasureTextSize("....", TxBArquivo.FontFamily, TxBArquivo.FontStyle, TxBArquivo.FontWeight, TxBArquivo.FontStretch, TxBArquivo.FontSize);

                var itextSize = new Size(extensionSize.Width + dotSize.Width, extensionSize.Height + dotSize.Height);
                var sb = new StringBuilder();
                foreach (var @char in dlg.FileName)
                {
                    sb.Append(@char);
                    var format = string.Format("{0}....{1}", sb, extension);
                    itextSize = MeasureTextSize(format, TxBArquivo.FontFamily, TxBArquivo.FontStyle, TxBArquivo.FontWeight, TxBArquivo.FontStretch, TxBArquivo.FontSize);
                    if (TxBArquivo.ActualWidth < itextSize.Width)
                    {
                        sb.Remove(sb.Length-1, 1);
                        passouDoTamanho = true;
                        break;
                    }
                }
                TxBArquivo.Text = passouDoTamanho ? string.Format("{0}....{1}", sb, extension) : dlg.FileName;
            }
        }

        public static Size MeasureTextSize(string text, FontFamily fontFamily, FontStyle fontStyle, FontWeight fontWeight, FontStretch fontStretch, double fontSize)
        {
            var typeface = new Typeface(fontFamily, fontStyle, fontWeight, fontStretch);
            var ft = new FormattedText(text, CultureInfo.CurrentCulture, FlowDirection.LeftToRight, typeface, fontSize, Brushes.Black);
            return new Size(ft.Width, ft.Height);
        }
    }
}