com/templates/repo/issue/view_content/attachments.tmpl

{{range .Attachments}}
  <a target="_blank" rel="noopener noreferrer" href="{{AppSubUrl}}/attachments/{{.UUID}}">
    {{if FilenameIsImage .Name}}
      <img class="ui image" src="{{AppSubUrl}}/attachments/{{.UUID}}" title='{{$.ctx.i18n.Tr "repo.issues.attachment.open_tab" .Name}}'>
    {{else}}
      <span class="ui image octicon octicon-desktop-download" title='{{$.ctx.i18n.Tr "repo.issues.attachment.download" .Name}}'></span>
    {{end}}
  </a>
{{end}}