UploadMeta.extra carries backend-specific /upload fields through the spool
sidecar; post_recording picks the MIME from the file suffix so a spooled
WAV is re-sent as audio/wav; call_recorder.start(capture=False) runs only
the spool, for a backend that records without PulseAudio.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>