Enable app-to-local sync of ARW/PSB/PSD/CR2/NEF files on Web UI using S3

Steps to Reproduce:

1. Start Nuxeo Drive with account linked to Web UI instance using the Amazon S3 binary manager.
2. Navigate to Web UI and create a Workspace; enable syncing on this workspace.
3. Confirm that the Workspace is replicated in the local Nuxeo Drive directory.
4. In the Workspace, import files of the following types using drag-and-drop: NEF/ARW/CR2/PSD/PSB (tested with both "Create" and "Add Properties")
5. Observe that the source files are not appearing in the local synced folder. 

Note that when viewing Document metadata via JSON/XML export, the digestAlgorithm is consistently null. In Nuxeo Drive debug.log entries associated with the attempted sync, I observed the following line:

WARNING  nxdrive.engine.watcher.remote_watcher Ignoring unsyncable document {...} because of unknown digest