When a user sends a file to your bot, the file is first uploaded to the user's OneDrive for Business storage. Your bot will then receive a message activity notifying you of the user upload. The activity will contain file metadata, such as its name and the content URL. You can directly read from this URL to fetch its binary content.
Ref:
https://learn.microsoft.com/en-us/microsoftteams/platform/resources/bot-v3/bots-files#message-activity-with-file-attachment-example
Thanks,
Prasad Das
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