The Media Viewer's file names are now designed to not include a file extension, to reduce visual clutter (e.g. we now show 'Goldfish' instead of 'Goldfish.jpg'), as seen on this design mockup (1) and screenshot. (2)
But users need a quick way to copy the entire file name from the Media Viewer, so they can quickly insert a file in an article (e.g. 'Goldfish.jpg').
To that end, our design team recommends we show the full name with file extension when the user hovers over the title. The user should be able to easily copy and paste that full name when they hover.
More info about Media Viewer on our project page (3).