You can find out what filetype Github has decided your file is by doing view-source on the webspage, and looking for this:

Github's filetype detection is performed by its linguist module. They have decent documentation for it at that link.

/bin/sh ⇒ exec another interpreter

Supposedly this is supposed to be detectable (see implementation), and use the argument right after the 'exec', but I haven't been able to get it to work myself.