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Thank you, Phong. I seem to be running into all sorts of Sandbox deficiencies. I will see what it takes to move to our production instance, but I had really hoped to wait until I knew my code was completely tested.

Sorry for the delayed response Andrew. I guess that you are testing this in the sandbox environment. Currently, there is problem with posting attachments to a sandbox team messaging. That is why you won't receive a notification event. This should work on production and here is an example of the event payload.

{"id":"3564165923xxxx","groupId":"8544346xxxx","type":"TextMessage","text":"","creatorId":"62288329016","addedPersonIds":null,"creationTime":"2022-02-09T16:07:30.886Z","lastModifiedTime":"2022-02-09T16:07:30.886Z","attachments":[{"id":"1594796859402","name":"hubspot-sproket.png","contentUri":"https://dl.mvp.ringcentral.com/file/159479685xxxx","type":"File"}],"activity":null,"title":null,"iconUri":null,"iconEmoji":null,"mentions":null,"eventType":"PostAdded"}