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Fix for: Slack Connector: Can't send attachments using utter_attachment
 https://github.com/RasaHQ/rasa_core/issues/762
RasaHQ/rasa_nlu,RasaHQ/rasa_core,RasaHQ/rasa_nlu,RasaHQ/rasa_core,RasaHQ/rasa_core,RasaHQ/rasa_nlu