File Upload Class

Regarding the use of an upload.php config file, the documentation says this:
If you prefer not to set preferences using the above method, you can instead put them into a config file. Simply create a new file called upload.php, add the $config array in that file. Then save the file in: config/upload.php and it will be used automatically. You will NOT need to use the $this->upload->initialize() method if you save your preferences in a config file.

To overrride the default error messages, simply copy the file upload_lang.php from the folder system/language/english to application/language/<your language>, and change the messages to what you want.