MY_ classed and extending native libraries

[eluser]sahanlak[/eluser]
Hello,

Because I'm having some issues with the Codeigniter upload class, I wanted to overwrite its is_allowed_filetype() function, I created a file called My_upload under application/libraries. But seems like it doesn't load properly, actually its not loading at all. I'm trying to extend the CI_Upload class, but for a test I created an empty class and echod something in __construct. At last I was able to load my class using auto load, I added my library name to autoload. So after I got it to load, I added my original code which extends the CI_Upload class. Now when I load my site, it says. Fatal error: Class 'CI_Upload' not found in......

I read the user guide and did as it say, but didn't work properly. Is there something that I have missed?

Thanks.