Steps to reproduce.

1. Run a test which attaches files including '%' character with JUnit Attachments Plugin.
e.g. https://gist.github.com/miyajan/703d52d1ec449a16f29ed31c6e89865e
2. Access to the page at the method level.
e.g. http://your-jenkins.example.com/job/attachments-test/1/testReport/com.example.miyajan/AttachmentsTest/testScreenshot/
3. Click a link '100%.png'.
-> 404 error.

It seems that the URL is not escaped.

current: http://your-jenkins.example.com/job/attachments-test/1/testReport/com.example.miyajan/AttachmentsTest/testScreenshot/attachments/100%.png
expected: http://your-jenkins.example.com/job/attachments-test/1/testReport/com.example.miyajan/AttachmentsTest/testScreenshot/attachments/100%25.png