attachments: fix opening the delete link directly

When cliking on the "Delete attachment" link, and opening the URL
in a new tab, the `DELETE /attachements/:id` will become
`GET /attachments/:id` – which will cause the `show` action to be
routed with an html format (instead of JS).

In that case, we don't want to throw an error at the user face.
Instead simply re-render the dossier page (if any).

Fix a long-standing error in Sentry.

spec/controllers/attachments_controller_spec.rb

@@ -28,6 +28,12 @@ describe AttachmentsController, type: :controller do
         end
       end
 
+      context 'when the user opens the delete link in a new tab' do
+        let(:format) { :html }
+
+        it { is_expected.to have_http_status(302) }
+        it { is_expected.to redirect_to(dossier_path(dossier)) }
+      end
     end
 
     context 'when not authenticated' do