Attachments are missing after Confluence migration to the cloud, error message "Attachment File Not Found"

Issue Summary

Attachments are lost when migrating using the Confluence Cloud Migration Assistant from server to cloud.

Environment

Confluence Cloud

Steps to Reproduce

1. Set up Confluence Server (5.10 to latest) with a production (i.e. non-H2) database
2. Set up a Space, create a page and attach multiple images to the page
3. Install the latest Confluence Cloud Migration Assistant)
4. Create and run a new plan to migrate the new space, and no users
5. Wait for the plan to be successfully completed
6. Review the imported data in the cloud

Expected Results

All images appear correctly

Actual Results

Attachments are broken.

Images do not display. Attachments may give an error with

`Attachment File Not Found`

`The attachment you were trying to download could not be found in the attachment file store. Either the file has been deleted manually or there was an error in removing the record of the attachment from Confluence.`

`If this attachment is no longer required you can delete it from Confluence on the attachments page. Otherwise, you will need to attach a new copy.`

`<< Back to attachments page`

The below exception is thrown in the log file:

   message: Requested attachments not found in Filestore, so serving it locally. attachmentId: xxxxxxxx 

Workaround:

1. Delete and reimport the space or
2. Use POST REST API call

    /wiki/rest/api/content/\{id}/child/attachment/\{attachmentId}/data 

   as mentioned on Confluence Cloud resti api documentation

For the second workaround, please contact our support team at https://support.atlassian.com/contact/ to export the broken attachments ID and name.

Note:

This might be a recurring of https://jira.atlassian.com/browse/MIG-33