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 picture:
To avoid this error, the admin has to decrypt all attachments before disabling the app. In some cases, users might still see the error picture, because the cache has stored this picture and is not requesting it to the server again.
To fix this, the user has to clean their cache and refresh the page. This will download the requested resource again from the server.