This limit was added in PHP 5.3.1, to avoid a type of DOS attack: temporary files exhaustion.
Added "max_file_uploads" INI directive, which can be set to limit the number of file uploads per-request to 20 by default, to prevent possible DOS via temporary file exhaustion. (Ilia)
(changelog for PHP 5.3.1)
You can increase this limit by changing the max_file_uploads
directive.
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