We'll respond shortly.
“S3 is giving us 400 timeout responses on every first upload for a request. All subsequent uploads for the same request seem to work fine.”
This turned out to be a bug in the HEAD version of Paperclip. They added a fingerprint method that creates a MD5 hash out of the original file, but the method forgets to rewind the original file when done. So S3 times out while waiting for data, but then rewinds the file before trying again. So it does ultimately get uploaded, but it was leading to ~10 second delays in the middle of the request.