Wpf drop event not firing

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C#

WPF

Hi,
I have a user control simulating water mark text box.
I need to allow drop text files on the user control.
The PreviewDragEnter event works fine. This is the event where i want to verify that the file that been drag is actually a text file.
The problem is that the drop event never fires.
I want to catch the event when the user release the mouse button and decide to drop the file into the user control.
I think it got something to do with the multiple controls inside the user control and the hierarchy.
I really don't care which control will fire the drop event as long as it actually fire.
This is the xaml (the control i want to target the drop event is name 'txt')

<UserControl x:Class="CrumbSearch.CustomControls.package_watermark"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             Height="45" Width="Auto" 
             FontFamily="B Yekan"             
             AllowDrop="true"
             Grid.IsSharedSizeScope="True"              
             xmlns:wpfwatermarktextbox="clr-namespace:WPFWaterMarkTextBox;assembly=WPFWaterMarkTextBox">
    <FrameworkElement.Resources>
        <ResourceDictionary>
            <FontFamily x:Key="simple_line_icon">/fonts/Simple-Line-Icons.ttf#simple-line-icons</FontFamily>
            <Style x:Key="SimpleTextBox" TargetType="{x:Type TextBox}">
                <Setter Property="AllowDrop" Value="True"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type TextBox}">
                            <Border Name="border" BorderBrush="{TemplateBinding BorderBrush}" 
                                    BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" SnapsToDevicePixels="True">
                                <ScrollViewer Name="PART_ContentHost" Focusable="False" Margin="2,10,0,0"
                                              AllowDrop="True"
                                              HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden" />
                            </Border>
                            <ControlTemplate.Triggers>
                                <Trigger Property="UIElement.IsFocused" Value="True">
                                    <Setter TargetName="border" Property="Border.BorderBrush" Value="Transparent" />
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
            <Style x:Key="SimplePasswordBox" TargetType="{x:Type PasswordBox}">
                <Setter Property="AllowDrop" Value="True"/>
                <Setter Property="Template">
                    <Setter.Value>
                        <ControlTemplate TargetType="{x:Type PasswordBox}">
                            <Border Name="border" BorderBrush="{TemplateBinding BorderBrush}" 
                                    BorderThickness="{TemplateBinding BorderThickness}" Background="{TemplateBinding Background}" 
                                    SnapsToDevicePixels="True">
                                <ScrollViewer Name="PART_ContentHost" 
                                              Focusable="False" HorizontalScrollBarVisibility="Hidden" VerticalScrollBarVisibility="Hidden" />
                            </Border>
                            <ControlTemplate.Triggers>
                                <Trigger Property="UIElement.IsFocused" Value="True">
                                    <Setter TargetName="border" Property="Border.BorderBrush" Value="#00000000" />
                                </Trigger>
                            </ControlTemplate.Triggers>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style>
        </ResourceDictionary>
    </FrameworkElement.Resources>
    <Border Name="border" BorderThickness="0.5" CornerRadius="4" MouseEnter="border_MouseEnter"             
            MouseLeave="border_MouseLeave">
        <FrameworkElement.Style>
            <Style TargetType="{x:Type Border}">
                <Setter Property="BorderBrush" Value="White" />
                <Setter Property="AllowDrop" Value="true" />
            </Style>
        </FrameworkElement.Style>
        <Grid Name="MainGrid" PreviewMouseLeftButtonDown="MainGrid_PreviewMouseLeftButtonDown"               
              LostFocus="MainGrid_LostFocus">
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="auto" />
                <ColumnDefinition/>
            </Grid.ColumnDefinitions>
            <Label Name="lbl2" Padding="5,0" FontSize="10" Content="Name" 
                   Margin="0,5,0,0"                   
                   HorizontalAlignment="Left" Foreground="White" FlowDirection="LeftToRight"
                   VerticalAlignment="Top" Height="13" Grid.IsSharedSizeScope="True" 
                   Opacity="0" Grid.Column="1" Background="{x:Null}" />
            <TextBlock Name="lbl" Text="WaterMark" HorizontalAlignment="Left" 
                       Foreground="#0EE7EC" FontSize="12"                        
                       Padding="5,5,5,0" FlowDirection="LeftToRight" FontFamily="Andalus"
                       TextWrapping="WrapWithOverflow" FontStyle="Italic"
                       Opacity="0.7" Grid.Column="1" 
                       VerticalAlignment="Center" Background="{x:Null}" />
            <TextBox Name="txt" Style="{DynamicResource SimpleTextBox}" FlowDirection="LeftToRight"                                                                        
                     AllowDrop="true" Background="{x:Null}" 
                     PreviewDragEnter="txt_PreviewDragEnter"
                     PreviewDrop="txt_PreviewDrop" Drop="txt_Drop"
                     TextWrapping="NoWrap" VerticalContentAlignment="Center" FontSize="14" 
                     VerticalScrollBarVisibility="Disabled" MaxLines="1" FontWeight="Medium" Grid.Column="1" 
                     BorderBrush="{x:Null}" TextChanged="TextBox_TextChanged"  Foreground="White"
                     PreviewGotKeyboardFocus="txt_PreviewGotKeyboardFocus" 
                     PreviewLostKeyboardFocus="txt_PreviewLostKeyboardFocus" />
            <PasswordBox Name="pass" Style="{DynamicResource SimplePasswordBox}" FontSize="14"                          
                         VerticalContentAlignment="Center" 
                         ScrollViewer.VerticalScrollBarVisibility="Disabled" Grid.Column="1" 
                         Visibility="Collapsed" BorderBrush="{x:Null}" 
                         Background="{x:Null}" PasswordChanged="PasswordBox_PasswordChanged" />
            <Label Name="LblIcon" Padding="0" FontFamily="{DynamicResource simple_line_icon}"                     
                   VerticalContentAlignment="Center" HorizontalContentAlignment="Center" 
                   Foreground="#FF6A6A6A" FontSize="14" Background="{x:Null}" />
        </Grid>
    </Border>
</UserControl>

What I have tried:

I tried to use protected override drop event but it want fire also. Only the drag event fires.