When storing a document with a property starting with an underscore, the document is stored as an attachment.

As I understand it, with the introduction of the meta object, underscore fields are no longer special nor protected. To reproduce, create a new doc with the JSON below either with an SDK or the web console.

{ "_foo" : "newDoc", "click": "to edit", "new in 2.0": "there are no reserved field names" }