I think you mean JIRA, not Confluence as you tagged this.
In JIRA, you add permissions with permission schemes. A permission scheme is added to a project, so all the permissions from a particular permission scheme are added to a project.
There are two permissions regarding deleting attachments:
So, edit the permission scheme that is associated with your project so that you can delete attachments.
You can find more detailed instructions here: https://confluence.atlassian.com/jira/managing-project-permissions-185729636.html
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