Jira administrators can enable this option to automatically rename newly added attachments with identical names as existing attachments. Note that Attachment Cleaner will currently only look at newly added attachments. An administrator can manually run the project attachment cleaner to clean older attachments.
You have an issue with an image attachment with the name yiraphic.png.
You upload a new attachment with the exact same name as the already existing attachment but which is not the exact same file.
Attachment Cleaner keeps track of newly added attachments and once every minute will look at and compare the newly added attachments. After Attachment Cleaner has run you'll notice that the original attachment has had its name changed.