ksh is a text/plain to, all this extension are text/plain:
'.ksh', '.pl', '.bat', '.h', '.c', '.txt', '.asc', '.text', '.pot', '.brf'.
The problem is: the code return the first of list:
return extensions[0]
So, I add one boolean parameter in method guess_extension called all_exts. Putting True in this parameter the method returns a tuple with all possible extensions.

I hope helped