".S" is not a valid extension when trying to add a .S tab

[per1234]

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.