The encryption method used for attachments and custom fields is:
Versions prior to 1.6.3
If the Jira Administrator disables the app, attachments will stay encrypted by default. Users will not be able to open an encrypted attachment, instead, they will see the following image:
To avoid this error, the Admin should decrypt all attachments before disabling the app. In some cases, users might still see the error image because it has been stored in their cache. To fix this, users should clear their cache and refresh the page. This will download the requested resource from the server again.