Attachments to HyperKitty archived messages can't be retrieved.
When an attachment link is 'clicked' in Hyperkitty, the resultant downloaded file contains only something like
<memory at 0x7fb18ccfea08>
When an attachment link is 'clicked' in Hyperkitty, the resultant downloaded file contains only something like
<memory at 0x7fb18ccfea08>