WeCom AI Bot sends file attachments with msgtype="appmsg", not
msgtype="file". Previously only file content was discarded while
the text title reached the agent.
Changes:
- _extract_text(): Extract appmsg title (filename) for display
- _extract_media(): Handle appmsg type with file/image content
Fixes#7750
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>