Example:

A new type could be defined, such as RealmFile. A RealmObject could have a RealmFile field or even a RealmList<RealmFile> field. Just like all other RealmObjects, this would simply be a proxy and wouldn't contain the actual file/binary data. To access or update the data in the RealmFile you would get an InputStream or OutputStream. Realm handles the rest in terms of syncing, caching, linking, etc. I know that Realm itself isn't really designed for arbitrarily large files, so the data contained in a RealmFile wouldn't necessarily need to be put into Realm. But it would be managed and handled by Realm as if it were. In terms of the backend, I don't know MongoDB's product offering well enough to know whether a file/blob storage backend exists that Realm could plug into.

It would be helpful if there were other attributes on the RealmFile as well, such as the current hash, which would get updated when writing data to the RealmFile. This would be helpful for querying based on a hash.

It would be great if revision history could be kept, either on individual RealmObjects, groups of RealmObjects, or the Realm as a whole. There could be a way to specify the fidelity of the revisions (e.g. every change or every X minutes) and a way to specify a TTL (e.g. last 30 days). If the Realm is synced, the local Realm could purge its cache of revisions older than the TTL and there could be an API to retrieve these when needed. Additionally, there could a (separate) TTL for permanent deletion of revisions as to keep the Realm from growing too large. Or, alternatively, a way to move revisions to cold storage.

A basic use case for this is editing a document and being able to view a revision history of the document and restore a previous revision. A document could be represented by a single RealmObject that contains several RealmObject and RealmList fields, so I imagine there would need to be a way to have the parent document RealmObject have a reference to a revision anytime any of its child RealmObjects change (of course, based on the fidelity).