File upload

Caution: File upload is always a major security issue !

Note: The following applies for a standard Apache-PHP-HTML-CSS-JS website, but much of it is valid in other cases too.
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File download

File download consists of front-end GUI and back-end code .

Caution: Never use include or require for uploaded files. Serve them as static or use file_get_contents(), readfile() etc.
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