You can now drag files onto work items to add as attachments.

This is a way to streamline adding attachments. Note that you will need to edit the attachment to add a reference - otherwise it will simply remain as a plain attachment.

Drag the file out of a file dialog box onto the work item where you wish to attach it - in this case an engagement letter. As you do the work item frame will turn blue:

Then release the mouse button and the file will upload automatically: