sorry guys, I solved this by myself. I don't know why outlook 2013 do not trigger the AttachmentContextMenuDisplay event.it does not. so I find the selected attachment(s) by another way.
when we right click on attachments. and click the ribbon button we added on the context menu. we have a param whose type is IRibbonControlPtr we can get the _AttachmentSelectionPtr by call get_Context method. that's it.
reference web site is here
http://msdn.microsoft.com/en-us/library/office/ff863628(v=office.15).aspx[
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