I am unable to set the ".spine" file extension to be opened by the Spine App on OSX 10.9 (mavericks).
This is what I see:
http://cl.ly/image/1i1K1n3g2L29
You can see that the Spine.app is disabled and no selectable.
It "may" be that on Mac the app needs to be configured to open files of a certain type:
http://stackoverflow.com/questions/3046 ... ven-format
I'm guessing you're using some sort of wrapper to get the app to run on Mac, so it may be that this wrapper can be configured to open files of given type.. not sure.