Making a WPF TextBox binding fire on each new character?

You have to set the UpdateSourceTrigger property to PropertyChanged

<TextBox Text="{Binding Source={StaticResource fileLister}, Path=Path, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
         Height="25" Margin="12,12,12,0" VerticalAlignment="Top"/>

In your textbox binding, all you have to do is set UpdateSourceTrigger=PropertyChanged.