or maybe you can combien button from this post [New... textfile > How to auto-select it?)
and add "rename inline=ext" at the end to select extension.
But i think rename inline=ext don't work anymore, rename dialog open 
or maybe you can combien button from this post [New... textfile > How to auto-select it?)
and add "rename inline=ext" at the end to select extension.
But i think rename inline=ext don't work anymore, rename dialog open