Rainbows! kills requests that exceed it's client_max_body_size silently, like a ninja. You won't even know it happened.
Turns out Rainbows had a configuration option called client_max_body_size that defaulted to 1 MB The option is documented here
If this options is on, Rainbows will kill large requests silently. You might not know it's breaking unless you run something in front of it.
Rainbows! do # let nginx handle max body size client_max_body_size nil end