def jsonString = """{"Files": [
   {"filepath": "input/file_29112017d.csv"},
   {"filepath": "input/file_29112017d.log"},
   {"filepath": "input/file_29112017d.ini"},
   {"filepath": "output/file_29112017d.xml"},
   {"filepath": "output/file_29112017d.csv.trc"}
]}"""
def json = new groovy.json.JsonSlurper().parseText(jsonString)
def filePath = json.Files.collect { element -> element.find{it.value.endsWith('csv')} }.findAll{it}.first().value
println filePath

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