3LO API support for get attachments method

Issue Summary

3LO apps cannot utilize the method to fetch attachments (ref: https://developer.atlassian.com/cloud/confluence/rest/api-group-content---attachments/#api-api-content-id-child-attachment-get). When a 3LO app does try to invoke that call, a 401 error is returned stating "Token is missing required scopes: confluence:atlassian-external"; however, that is not a recognized scope for 3LO apps.

Steps to Reproduce

1. Create a 3LO application
2. Authorize 3LO app as a user
3. Make get attachment API call to instance with bearer token to 3LO endpoint (i.e. https://api.atlassian.com/ex/confluence/<app-id>/wiki/rest/api/content/<page-num>/child/attachment)

Expected Results

200 response code with attachment data.

Actual Results

401 response code.

Workaround

There are no workarounds. One would have to either use basic auth (w/API tokens) or Connect framework.