Grails spock Testing with File class

I have a method that create a file based in a content copied from an another file. Like below

private cloneBaseFile(fileName, ddi, ddd){


    def config = grailsApplication.config.kicksim.fileConstants


    String baseFileContents = new File(config.baseFile).getText('UTF-8')

    def help = handleString("${ddd}.${ddi}")

    baseFileContents = baseFileContents.replaceAll("DDDDDI", help);

    def f1= new File(fileName)
    f1 << baseFileContents

    return fileName
}

I'd like to know how to unit test it.