File_Upload_Upgrader::__construct() WordPress Method

	public function __construct( $form, $urlholder ) {

		if ( empty( $_FILES[ $form ]['name'] ) && empty( $_GET[ $urlholder ] ) ) {
			wp_die( __( 'Please select a file' ) );
		}

		// Handle a newly uploaded file. Else, assume it's already been uploaded.
		if ( ! empty( $_FILES ) ) {
			$overrides = array(
				'test_form' => false,
				'test_type' => false,
			);
			$file      = wp_handle_upload( $_FILES[ $form ], $overrides );

			if ( isset( $file['error'] ) ) {
				wp_die( $file['error'] );
			}

			$this->filename = $_FILES[ $form ]['name'];
			$this->package  = $file['file'];

			// Construct the attachment array.
			$attachment = array(
				'post_title'     => $this->filename,
				'post_content'   => $file['url'],
				'post_mime_type' => $file['type'],
				'guid'           => $file['url'],
				'context'        => 'upgrader',
				'post_status'    => 'private',
			);

			// Save the data.
			$this->id = wp_insert_attachment( $attachment, $file['file'] );

			// Schedule a cleanup for 2 hours from now in case of failed installation.
			wp_schedule_single_event( time() + 2 * HOUR_IN_SECONDS, 'upgrader_scheduled_cleanup', array( $this->id ) );

		} elseif ( is_numeric( $_GET[ $urlholder ] ) ) {
			// Numeric Package = previously uploaded file, see above.
			$this->id   = (int) $_GET[ $urlholder ];
			$attachment = get_post( $this->id );
			if ( empty( $attachment ) ) {
				wp_die( __( 'Please select a file' ) );
			}

			$this->filename = $attachment->post_title;
			$this->package  = get_attached_file( $attachment->ID );
		} else {
			// Else, It's set to something, Back compat for plugins using the old (pre-3.3) File_Uploader handler.
			$uploads = wp_upload_dir();
			if ( ! ( $uploads && false === $uploads['error'] ) ) {
				wp_die( $uploads['error'] );
			}

			$this->filename = sanitize_file_name( $_GET[ $urlholder ] );
			$this->package  = $uploads['basedir'] . '/' . $this->filename;

			if ( 0 !== strpos( realpath( $this->package ), realpath( $uploads['basedir'] ) ) ) {
				wp_die( __( 'Please select a file' ) );
			}
		}
	}