WordPress at a glance
action is not described

post-html-upload-ui action-hook . WP 2.6.0

Fires after the upload button in the media upload interface.

Usage

add_action( 'post-html-upload-ui', 'action_function_name_5673' );
function action_function_name_5673(){
	// action...
}

Where the hook is called

media_upload_form()
wp-admin/includes/media.php 1969
do_action( 'post-html-upload-ui' );

Where the hook is used (in WP core)

wp-admin/includes/admin-filters.php 26
add_action( 'post-html-upload-ui', 'media_upload_html_bypass' );