Extension Builder creates malformed icon file
I built a new extension without specifying an icon.
In the root directory of the .roboFontExt package, RoboFont wrote a file with the name 'Icon\r'. (ie. Icon plus the non-displaying carriage return character).
Not a big deal, but it's a bug.
if you have assets (almost everything which isn't a python file) store them into a resource folder and connect it in the extension bundle.
to retrieve assets see http://doc.robofont.com/api/mojo/mojo-extensions/