I don't know if it's a bug or what, but an error message will pop-up when trying to double click on a .r extension file.

to fix it:

cd to ~/.local/share/applications

nano defaults.list

add a line with the MIME Type that you are having problems opening and the application that you want to use to open it.

For example:

I added this line to open .r files:

application/x-extension-r=kde4-kwrite.desktop

cheers.