N/file Module
File.folder
Property Description: The internal ID of a file’s folder within the NetSuite File Cabinet. Before you upload a file to the NetSuite File Cabinet with File.save(), you must set its File Cabinet folder with the folder property.
Type: number | string
Supported Script Types: Server-side scripts For more information, see SuiteScript 2.0 Script Types.
Module: N/file Module
Since: 2015.2
Search NetSuite - https://system.netsuite.com/app/help/helpcenter.nl?search=File.folder

// Code Example 1

//Add additional code

//...

var fileObj = file.create({

    name: 'test.txt',

    fileType: file.Type.PLAINTEXT,

    contents: 'Hello World\nHello World'

    });

fileObj.folder = 30;

var fileId = fileObj.save();

//...

//Add additional code



//SOURCE: https://system.netsuite.com/app/help/helpcenter.nl?fid=section_4229265810.html