Thank you for the additional information provided.
I am afraid that the Kendo Upload widget does not officially support drag-and-drop upload of messages from Outlook. Moreover, while reviewing the scenario in question, I noticed that IE 11 and Firefox do not upload any messages dragged from Outlook. Those are uploaded in Edge with no changes in Outlook, while in Chrome, the uploaded messages are removed from Outlook.
Taking into account that the discussed functionality is not present at all for some browsers and the fact that it is not officially supported by the Kendo Upload, I would recommend you to avoid relying on that.
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