Manual:Uploading/gu
Uploading is done by Special:Upload or API:Upload. There are two types of uploads with separate user rights and log actions. The "upload" event uploads a new file; the "overwrite" event ("reupload" user right) uploads a new version of an existing file.
Database
The image.img_sha1 and image.img_timestamp data are stored in logging.log_params as a PHP-serialized array.