Sounds like they discovered a feature. This was initially designed to drag and drop file attachments from the local operating system. Dragging artifacts from an email application directly into a browser tab normally results in odd and unreliable behavior so I don’t think this was an intended function. The fact that it works in 2 browsers is interesting. Perhaps IE and Firefox tie into the OS more in order to allow that file to transfer across without a step in between like dropping on the desktop. I am honestly not sure.