GoError: "open" function is only available to the init code (aka global scope), see https://docs.k6.io/docs/test-life-cycle for more information

I have created the K6 test script using open api generator but while running it getting following error for the requests with input as multipart files
GoError: "open" function is only available to the init code (aka global scope), see https://docs.k6.io/docs/test-life-cycle for more information