Did you recently upgraded to the 8.15 version of Jira.
The issue has been found saying attachments are not able to view in Jira Server.
If so please check whether these files are present in installation directory folder jira/atlassian-jira/WEB-INF/classes/com/atlassian/jira/web/servlet
If not present add it from the previous backup copying each file to the location I have specified.
The link to the files is here
If it did not solve, please check this link from the knowledge base here https://confluence.atlassian.com/jirakb/existing-attachments-will-not-open-after-migrating-jira-server-785453577.html
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