I tried to use following code to get attachment from reponse as text in Groovy.
def testStep = testRunner.testCase.getTestStepByName("getData")
def response = testStep.testRequest.response
def ins =  response.attachments[0].inputStream
log.info(ins);
It contains some binary information too, so it is not fully human readable, but got following in output:
java.io.ByteArrayInputStream@5eca74
