Drag&Drop files to Outlook add-in

I have a problem with Outlook 365 add-in we are developing. The add-in should accept files dragged from user desktop. The web page is working fine with Drag&Drop in any browser: IE, Edge, Opera.

Same web page in add-in accepts only files dragged from Microsoft Office Application and doesn't accept anything from Outlook, user desktop or any other website or app.

Is there any special security setting in place?