OS-specific extensions are not default for exporting
Operating system or device - Godot version: win7x64, godot 2.1.1
Issue description (what happened, and what was expected): Export to win/mac/linux, and the filename is always whatever the previous export was, even for a different OS. It should be no extension for linux, .zip for Mac (I think), and .exe for Windows.
Steps to reproduce: Export to the above with the standard templates installed.
Link to minimal example project (optional but very welcome):