Drag-drop into Outlook asynchronously

I am trying to drag files from my own window into outlook and I got this ([login to view URL]). This brilliant article demonstrates how to create a customized DataObject implementing delayed data extract on drop/paste and asynchronous data trnasfer on background thread. It worked perfectly untill I tried to drop a file into Outlook 2016. It still works but the UI is not responsive despite the fact that my dataobject has "IsAsynchronous" set to true. After debugging I found when dropped to Outlook [login to view URL] is not called and [login to view URL] is called on the UI thread. My question is does Outlook support IAsyncOperation? If it does what am I missing? If it does not support IAsyncOperation, is there a workaround or different solution? Regards Xiao