For security reasons, you can't load general files from the disk. If you want to store some data on disk, you can use the PPAPI File I/O routines to read and write from files in the domain-isolated filesystem Chrome maintains (shared with the HTML5 filesystem APIs).
Sadly, this will be more complicated than the simple C++ file I/O routines you're probably used to. I believe there should be some examples of doing file I/O in the Native Client SDK.