WordPress at a glance
action is not described

pre-upload-ui action-hook . WP 2.6.0

Fires just before the legacy (pre-3.5.0) upload interface is loaded.

Usage

add_action( 'pre-upload-ui', 'action_function_name_2430' );
function action_function_name_2430(){
	// action...
}

Where the hook is called

media_upload_form()
wp_print_media_templates()
wp-admin/includes/media.php 1852
do_action( 'pre-upload-ui' );
wp-includes/media-template.php 213
do_action( 'pre-upload-ui' );

Where the hook is used (in WP core)

Does not used.