If the attachments directory has the wrong permissions, you may be unable to download or upload attachments and thumbnails. You will see the issue in the ftrack log as well:
IOError: [Errno 13] Permission denied: u'/opt/ftrack_config/data/attachments/a/6/1/1/d850-4936-11e6-a61e-0026b94bafec'
To fix the issue, follow the instructions for restoring permissions.