Fallback to Atlassian's S3 is never reached when a preloaded attachment is missing from Xray's storage

When the app-data preload runs, Xray copies the preloaded attachments from Atlassian's S3 into its own temporary storage. If Atlassian never sends the preload event, or the preload job itself fails, those files are not present in Xray's temporary storage.

During the project migration, the attachment migrator is supposed to handle this. It first tries to migrate the attachment from Xray's own storage, and if it is not found there, it falls back to fetching it directly from Atlassian's S3.

However, currently that fallback is never reached. When the attachment is missing from Xray's storage, the lookup throws instead of reporting that the file was not found, so the error propagates immediately and the fallback is skipped. The attachment fails to migrate with a "File does not exist" error pointing at Xray's own preload prefix.

The result is that the file is neither exported from Data Center (since it was treated as preloaded) nor recovered from Atlassian's S3, so it is lost with no fallback path.

Impact

Any migration where the preload fails or the preload event is not received will silently lose the affected attachments. Nothing surfaces the problem until after the migration completes.