A Content-Type form field such as "\n" was copied verbatim into the
multipart part header sent to the volume server, which broke the
framing and surfaced as a Content-MD5 mismatch and a 500. Form fields
promoted to request headers must be valid header values, as on a PUT;
otherwise answer 400 MalformedPOSTRequest.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>