bp_document_attach_document_to_message( $message )

function bp_document_attach_document_to_message( &$message ) {

	if ( bp_is_messages_document_support_enabled() && ! empty( $message->id ) && ! empty( $_POST['document'] ) ) {

		remove_action( 'bp_document_add', 'bp_activity_document_add', 9 );
		remove_filter( 'bp_document_add_handler', 'bp_activity_create_parent_document_activity', 9 );

		$document_list = $_POST['document'];
		$document_ids  = array();

		foreach ( $document_list as $document_index => $document ) {
			$title         = ! empty( $document['name'] ) ? $document['name'] : ' ';
			$attachment_id = ! empty( $document['id'] ) ? $document['id'] : 0;
			$menu_order	   = ! empty( $document['menu_order'] ) ? $document['menu_order'] : 0;

			$attachment_data = get_post( $document['id'] );
			$file            = get_attached_file( $document['id'] );
			$file_type       = wp_check_filetype( $file );
			$file_name       = basename( $file );

			$document_id = bp_document_add(
				array(
					'attachment_id' => $attachment_id,
					'title'         => $title,
					'privacy'       => 'message',
					'error_type'    => 'wp_error',
					'menu_order'    => $menu_order,
				)
			);

			if ( ! empty( $document_id ) && ! is_wp_error( $document_id ) ) {
				$document_ids[] = $document_id;

				// save document meta.
				bp_document_update_meta( $document_id, 'file_name', $file_name );
				bp_document_update_meta( $document_id, 'thread_id', $message->thread_id );
				bp_document_update_meta( $document_id, 'extension', '.' . $file_type['ext'] );

				// save document is saved in attachment.
				update_post_meta( $attachment_id, 'bp_document_saved', true );
			}
		}

		$document_ids = implode( ',', $document_ids );

		// save document meta for message.
		bp_messages_update_meta( $message->id, 'bp_document_ids', $document_ids );

		add_action( 'bp_document_add', 'bp_activity_document_add', 9 );
		add_filter( 'bp_document_add_handler', 'bp_activity_create_parent_document_activity', 9 );
	}
}