GHSA-v6f4-jwv9-682w: class.upload.php allows cross-site scripting attacks via uploaded files

As a simple library, class.upload.php does not perform an in-depth check on uploaded files, allowing a stored XSS vulnerability when the default configuration is used.

Developers must be aware of that fact and use extension whitelisting accompanied by forcing the server to always provide content-type based on the file extension.

The README has been updated to include these guidelines.