...
- Attachment synchronization is enabled on Contract (IssueSYNC 1.x.x).
- 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. Use corresponding trigger.
...
Relevant information will appear in Synchronization History Section on issue screen. 
If receiving attachements is disabled in configuration then you will get follwoing status:
