Thumbnail export should be disabled for folderish document types, otherwise you get this:
{"entity-type":"exception","code":"org.nuxeo.ecm.webengine.model.exceptions.WebResourceNotFoundException","status":404,"message":"No Blob was found for rendition 'thumbnail'"}
UPDATE: seems that this is in the pipeline already...
https://jira.nuxeo.com/browse/NXP-17308
