Issue Summary

After upgrading to Jira Data Center 11.x, attachments containing a semicolon ( in the filename cannot be downloaded. The upload succeeds without error, but clicking download results in “Ouch! We can't load the image.“ error and the request fails with 400 error code. The semicolon is not part of the officially restricted character set (\, /, ", %, :, $, ?, *) per https://confluence.atlassian.com/adminjiraserver/configuring-file-attachments-1027142528.html, so Jira correctly allows the upload.

This may be a regression introduced by the fix for https://jira.atlassian.com/browse/JRASERVER-76634, which tightened special character handling for attachments.

Steps to Reproduce

1. Open any issue with the attachment section visible.
2. Upload a file with a semicolon in the filename (e.g., test1;test2.txt) via drag-and-drop or the Browse dialog.
3. Upload succeeds — the attachment appears in the Attachments section.
4. Click on the attachment to download it.

Expected Results

The attachment is downloaded successfully.

Actual Results

The individual attachment download fails with HTTP Status 400 – Bad Request. If the attachment is an image, the thumbnail displays "Ouch! We can't load the image."

Workaround

• “Download all attachments” option still works and can be used as a workaround.