As per JRASERVER-34927: Automatically include attachment link in attachment comment (fixed in Jira Server and Data Center 6.5-OD-06, 7.0.0) which was rolled out a few years ago, each time an attachment is added to an issue, Jira will automatically add the link/thumbnail in the comment field.

Suggestion

It would be great if users can choose to disable this feature (make it optional at a certain level, let's say per project or globally). If it is disabled, there will be no comment generated.

Workaround

Use the attachment option from the comment panel on the issue view screen instead:

1. In the comment panel at the bottom of the page, click on the paperclip icon to pull the attachment option. Click + Browse to attach a file.
   
2. Wait for a while until the file is attached successfully. A notification on the top right will appear indicating file is attached.
   
3. Verify under the Attachments field that the file is already attached.
   
4. In the comment panel, click Cancel so no comment will not be added.