The Signal adapter was passing image_paths, audio_path, and document_paths
to MessageEvent.__init__(), but those fields don't exist on the dataclass.
MessageEvent uses media_urls (List[str]) and media_types (List[str]).
Changes:
- Replace separate image_paths/audio_path/document_paths with unified
media_urls and media_types lists (matching Discord, Slack, etc.)
- Add _ext_to_mime() helper to map file extensions to MIME types
- Use Signal's contentType from attachment metadata when available,
falling back to extension-based mapping
- Update message type detection to check media_types prefixes
Fixes TypeError: MessageEvent.__init__() got an unexpected keyword
argument 'image_paths'
d04b9f4dc5