To do so with a Messenger Plus! Plugin you need to do the following
Note this is in Visual Basic
code:
Private objMessengerAPI As Object
Private Const M_ContactProperty_DP As Long = 2
Option Explicit
Public Function Initialize(ByVal nVersion As Long, _
ByVal sUserEmail As String, _
ByVal oMessenger As Object) _
As Boolean
Initialize = True
Set objMessengerAPI = oMessenger
objMessengerAPI.MyProperty(M_ContactProperty_DP) = "<filename>"
End Function