What are core files that handle jpg image uploads

Can anyone point me in the directions of the core files that handle image uploads specifically jpg images. I’m try to troubleshoot a trac ticket, but having trouble finding out whether it’s WordPress that handles the upload and interpretation of images or if WP just triggers a process on the server and uses GD or ImageMagick to do the work. If anyone knows how that works or knows of any advance documentation on the subject it would be much appreciated.

I’ll carry on plodding through the codex if no one has any suggestions, thanks for reading this. 🙂