try: var csvFile = file.create({ name: 'data.csv', contents: codesList, folder: 123, fileType: 'PLAINTEXT', encoding: 'UTF-8' });

A csv file would be easy. It's just plain text you can generate in a script. I suppose you use the N/search module to get the data for your list to view in the Suitelet? Use the same data, iterate over the list, and create a string by concatenating the cells with ',' or '\t' (prefer tsv instead of csv). After each…

The person who drags the file onto the record, need to have at least edit permissions on the record itself, as besides dropping the file in the file cabinet, it also need to add it to the record. Therefore you need edit permissions. That's why you get the error. Check your roles and permissions in NetSuite.