Symptom
Attachments are not synchronized.
Explanation
If the field "Attachment" is hidden in a project configuration, Backbone cannot synchronize attachments. It's still possible to upload attachments to issues but attachments are not visible through the API. This behavior is related to a bug in Jira Server and Jira Cloud.
Solution
Make the "Attachment" field visible in the project settings. Perform a resync with attachments afterwards, to make sure attachments uploaded till now are synchronized again.