Is there a ressource that would let me access an attachment in a google hangouts chat, I can see the url to get a preview, for instance : https://chat.google.com/downloads?attachment_token=TOKEN&content_type=image%2Fpng
But I failed to find in the rest api reference anything relevant, I thought it would have been in the message get : https://developers.google.com/hangouts/chat/reference/rest/v1/spaces.messages/get
As of now Hangouts Chat currently does not have this feature as the preview link is generated internally. The best thing to do is to look for an existing feature request or file a new FR via https://issuetracker.google.com/issues/new?component=350158&template=1047215 and hope that it gets enough upvotes.