Jira uses to manage the comments come from the mail client (i.e. Outlook) the following way:

  • it automatically collects all images to the issue attachments
  • it replaces the image with the [cid:] reference in the comment

That's why you can see something like that in the comment:

cid example

If you would like to avoid such attachment references to be created in the comments, tick the Turn on filtering of [cid:] messages from comments checkbox in Jira Administration > CRM > Direct Mail: