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* Attachment download needed an expand operation. Added that.
* Added test coverage with gock. Tested with manual backup of posts which contain attachments(embedded/non-embedded). We can't add e2e tests with attachments, since API to create new conversations requires delegated access.
* Note that https://github.com/alcionai/corso/issues/4991 is still an open item. We don't have a resolution for it right now, since attachment endpoint requires delegated token. Defaulting to let the backup fail for "too many attachments" error case. We don't know yet if we'd see that with group mailboxes, and whether it'd be the same error that we saw with exchange (recurring 503s).
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#### Does this PR need a docs update or release note?
- [x] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No
#### Type of change
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- [ ] 🌻 Feature
- [x] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup
#### Issue(s)
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* #<issue>
#### Test Plan
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- [x] 💪 Manual
- [x] ⚡ Unit test
- [ ] 💚 E2E
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