Attachments can synchronize one-way or two-way. It is a matter of configuration.

When attachments are synchronized?

• Attachment synchronization is enabled on How to configure synchronization.
• Issue is currently synchronized
• Each time you add a new attachment to the issue (you can drag&drop it to the panel or use Attach File operation from menu).
• Issue is not yet synchronized
• If issue already has some attachments (maybe synchronization is triggered by Create Remote Action, Workflows or JQL) it is possible to synchronize all existing attachments with issue synchronization trigger.