We are busy working on a deterministic way to sync attachments down to the devices using special sync rules, but at the moment attachments are not synched automatically, need to be fetched on demand and therefore not available offline (at least not deterministically, they are available for a limited time if they have already been fetched on demand).
For now then you will need to basically do a “online/offline” check before trying to show the attachments. Please take a look at this post for suggestions on how to implement an online/offline check