As an integration consultant and SAP Trainer (CLD900) I see many developers using groovy scripts to store the payload to the MPL (message processing logs).
This is not a reliable approach to log payloads in production tenants and should be used for testing and special purpose only, not as a general approach.SAP has added a circuit breaker to stop writing MPL attachments if 1 GB data is stored in 24 hours, see SAP note 2593825.
Use the correct integration pattern instead: Persist Message! This puts the payload (incl. headers and attachment(s) into the message store (database) for 90 days.
As SAP does not (yet) provide a UI to open those payloads from the tenant management node, we are delivering the tool for free in the mean time: WHINT MessageStore Viewer.
Feel free to download it and deploy on your CPI tenant within minutes.