For the REST API: http://localhost:8080/nuxeo/api/v1/id/ + document Id
With HTTP header: X-NXProperties: *
The url to a thumbnail comes out looking like
"thumb:thumbnail": { "data": "http://localhost:8080/nuxeo/nxfile/default/3530d4eb-ff8d-42f8-938f-2bc67421c6a6/thumbnail:thumb:thumbnail/Africa_and_Europe_from_a_Million_Miles_Away.png", "digest": "12e84fd637b652c54dda2533e2456287", "digestAlgorithm": "MD5", "encoding": null, "length": "178250", "mime-type": "image/png", "name": "Africa_and_Europe_from_a_Million_Miles_Away.png" },
If one debugs at https://github.com/nuxeo/nuxeo/blob/master/nuxeo-core/nuxeo-core-io/src/main/java/org/nuxeo/ecm/core/io/marshallers/json/document/DocumentPropertyJsonWriter.java#L199 it is apparent that the BlobProperty name for file:content is content, but for thumb:thumbnail it is thumb:thumbnail.