How to handle drag/drop without violating MVVM principals?

Question

Currently I have in my XAML

All of

Answer 1

This is just an additional answer that ports @Asheh's answer's to VB.NET for VB developers.

Imports System.Windows

Interface IFileDragDropTarget

    Sub OnFileDrop(ByVal filepaths As String())

End Interface

Public Class FileDragDropHelper

    Public Shared Function GetIsFileDragDropEnabled(ByVal obj As DependencyObject) As Boolean
        Return CBool(obj.GetValue(IsFileDragDropEnabledProperty))
    End Function

    Public Shared Sub SetIsFileDragDropEnabled(ByVal obj As DependencyObject, ByVal value As Boolean)
        obj.SetValue(IsFileDragDropEnabledProperty, value)
    End Sub

    Public Shared Function GetFileDragDropTarget(ByVal obj As DependencyObject) As Boolean
        Return CBool(obj.GetValue(FileDragDropTargetProperty))
    End Function

    Public Shared Sub SetFileDragDropTarget(ByVal obj As DependencyObject, ByVal value As Boolean)
        obj.SetValue(FileDragDropTargetProperty, value)
    End Sub

    Public Shared ReadOnly IsFileDragDropEnabledProperty As DependencyProperty = DependencyProperty.RegisterAttached("IsFileDragDropEnabled", GetType(Boolean), GetType(FileDragDropHelper), New PropertyMetadata(AddressOf OnFileDragDropEnabled))

    Public Shared ReadOnly FileDragDropTargetProperty As DependencyProperty = DependencyProperty.RegisterAttached("FileDragDropTarget", GetType(Object), GetType(FileDragDropHelper), Nothing)

    Shared WithEvents control As Windows.Controls.Control
    Private Shared Sub OnFileDragDropEnabled(ByVal d As DependencyObject, ByVal e As DependencyPropertyChangedEventArgs)
        If e.NewValue = e.OldValue Then Return
        control = TryCast(d, Windows.Controls.Control)
        If control IsNot Nothing Then
            AddHandler control.Drop, AddressOf OnDrop
        End If
    End Sub

    Private Shared Sub OnDrop(ByVal _sender As Object, ByVal _dragEventArgs As DragEventArgs)
        Dim d As DependencyObject = TryCast(_sender, DependencyObject)
        If d Is Nothing Then Return
        Dim target As Object = d.GetValue(FileDragDropTargetProperty)
        Dim fileTarget As IFileDragDropTarget = TryCast(target, IFileDragDropTarget)
        If fileTarget IsNot Nothing Then
            If _dragEventArgs.Data.GetDataPresent(DataFormats.FileDrop) Then
                fileTarget.OnFileDrop(CType(_dragEventArgs.Data.GetData(DataFormats.FileDrop), String()))
            End If
        Else
            Throw New Exception("FileDragDropTarget object must be of type IFileDragDropTarget")
        End If
    End Sub
End Class