Previously, mp4 video files were sent with msg_type "file", which meant
users had to download them to play. Feishu requires msg_type "media" for
audio and video files to enable inline playback in the chat.
Changes:
- Add _VIDEO_EXTS constant for video file extensions (.mp4, .mov, .avi)
- Use msg_type "media" for both audio (_AUDIO_EXTS) and video (_VIDEO_EXTS)
- Keep msg_type "file" for documents and other file types
The upload_file API already uses file_type="mp4" for video files via the
existing _FILE_TYPE_MAP, so only the send msg_type needed fixing.
