Actual Problem:

If you disable thumbnail generation system wide (Administration > System > Attachments), when attaching an image to JIRA the image will not be displayed as JIRA adds the following to the comment:

!imagename.jpg|thumbnail!

Steps to reproduce:

1. Disable Thumbnails
2. Add an image by copying and pasting it or using the Attachment > Browse option in the Text Editor:
   
3. The comment will not display the image:
   

Expected Behavior:

• Display the image without adding a flag that will not work as it's disable system-wide
  OR
• Display the image as an attachment link instead.

Workaround:

1. Attach the image and then click on the Attachment option again to add the image. This will include the image without the thumbnail flag.
2. Manually remove the |thumbnail from the comment.
3. Manually use attachment links.