The preflight audio transcription detection used camelCase `contentType`
but Discord's APIAttachment type uses snake_case `content_type`. This
caused `hasAudioAttachment` to always be false, preventing voice message
transcription from triggering in guild channels where mention detection
requires audio preflight.
Fixes#30034
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>