Using Arduino IDE 1.6.12 Hourly Build 2016/09/14 08:20 with Windows 7 64 bit - Click the tab menu button(downward pointing triangle) > New Tab - Name for new file: foo.S - Click the "OK" button - "Problem with rename" dialog pops up with the message `".S" is not a valid extension`. This doesn't occur with Arduino IDE 1.6.11 or previous. This doesn't occur with .s extension.