Symptoms

The thumbnails of the attached pictures are not visible in the issues although the 'Enable Thumbnails' field has been set 'ON' in Administration >> Global Settings >> Attachments. Only thumbnail can't be viewed, other attachments other than pictures such as .txt file all working properly. 

Cause

You don't have the 'Read' (Read attributes, Read extended attributes, Read Permission) permission for the JIRA_Home or JIRA_Home\data\ attachment folder.

Resolution

• • Give the 'write' permission for the current operating system user/admin.
  • Reload the page.
  • Windows: Right click on the folder, 'Properties > Security > Advanced > Choose user > Change Permission'
  • Linux: May refer to the following blog, Linux File & Folder Permissions
     You need to have the OS administration permission to do it.