I tried to use following code to get attachment from reponse as text in Groovy
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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