- Use asyncio.to_thread for file I/O to avoid blocking event loop
- Add 200MB upload size limit with early rejection
- Fix file handle leak by using context manager
- Use memoryview for upload chunking to reduce peak memory
- Add inbound download size check to prevent OOM
- Use asyncio.to_thread for write_bytes in download path
- Extract inline media_type detection to _guess_wecom_media_type()