Edit in App does not work when Confluence is integrated transparent SSO (and 2FA)

Issue Summary

If Confluence uses Two Factor Authentication, the Companion App cannot edit/save Office files when using the Edit With functionality. The error below is thrown when trying to open an Office file with the Companion app:

Excel cannot open the file XXXXX.xlsx because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

Environment

• Confluence 6.11.2
• Third party Two Factor Authentication

Steps to Reproduce

1. Install the Atlassian Companion App
2. Create a page
3. Upload an Excel file (or any Office file) to the page
4. Click on the file to show in Preview
5. Click Edit with Excel

Expected Results

The file opens in Excel and the user is able to edit the file and save it.

Actual Results

The Companion App tries to retrieve the attachment from the Confluence URL, but since the instance is behind 2FA, it cannot authenticate the request.

The Excel file becomes corrupt, and its contents contain tagging from the 2FA application denying the Companion App access to the Trusted URL.
 

Workaround

For the time being, the only way to work around this error is to disable the Atlassian Companion App. This can be done manually, but it requires downgrading a plugin which may not be compatible with newer versions of Confluence and is not supported. However, this has been tested and it works with up to Confluence 6.12 so far:

• Shutdown Confluence
• Copy the OfficeConnector-6.0.0.jar from an older version of Confluence (i.e. 6.10)
• Place it in the confluence_install/confluence/WEB-INF/atlassian-bundled-plugins/ folder
• From the same folder, move the OfficeConnector-7.0.3.jar to another folder as backup
• Clear the plugin cache
• Restart Confluence

You can then disable the Atlassian companion app functionality, and use 'Edit in office' moving forward.
Note this is only recommended if there is an SSO issue or if there is something in your environment that blocks the use of the companion app. For companion app troubleshooting, please reference this guide: Administering the Atlassian Companion App.