Update v2 upload interaction to match late-breaking changes

[ChrisLoer]

There's two parts here:

• Response values under ["data"]["attributes"] have been promoted to the top of the response
• first_layer_id has been renamed layer_id

I went ahead and made the PR myself to get a plugin build I could test with

[ChrisLoer]

(excuse my first broken push, I've gotten so used to using graphite I forgot how to use plain git)