Code:
Function SelectFile()
  Dim $objSHELLexec, $CMD, $objSHELL

  $SelectFile = ""
  $CMD = 'powershell.exe -NoProfile -STA -Command "Add-Type -AssemblyName System.Windows.Forms; ' +
         '$f = New-Object System.Windows.Forms.OpenFileDialog; ' +
         'if ($f.ShowDialog() -eq [System.Windows.Forms.DialogResult]::OK) { $f.FileName }"'
  $objSHELL = CreateObject("WScript.Shell")
  $objSHELLexec = $objSHELL.Exec($CMD)
  If Not $objSHELLexec.StdOut.AtEndOfStream
    $SelectFile = $objSHELLexec.StdOut.ReadLine()
  EndIf
EndFunction


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(... better days ahead)