The drag and drop functionality is determined by the browser and Outlook, which does not support the file system hooks that a browser must use to grab the file when presented with a file drop event. It's an Outlook design which limits the drop and drag ability that browsers rely on. 

 

In order to drag-and-drop a file into ServiceNow, you will need to first drag the file to your desktop and then drag it into the SN instance as this converts the file into something that is readable by the platform. 

 

 

Here are a couple of sites which discuss this limitation of Outlook: 

https://social.technet.microsoft.com/Forums/lync/en-US/f55628e6-36ab-444e-bfc0-57df0e323737/drag-and-drop-from-outlook-to-a-browser-based-application?forum=outlook 

https://stackoverflow.com/questions/17184369/upload-fails-when-user-drags-and-drops-attachment-from-email-client/17197224#17197224