The function isPrintable looks not deep enough into the registry to find the printable program.
Example: Windows 7 x64 JPEG File
In HKEY_CLASSES_ROOT\.jpg isn't a Subkey "shell\print\command", but it points to the Key "jpegfile" which hasn't a "shell\print\command", but a "shell\printto\command", which is also useful to determine, that a file is printable.