Fix ValueError in file_uploader.py by converting @gen.coroutine to async/await

Fix ValueError in file_uploader.py by converting @gen.coroutine to async/await

Fixes #3182

The file_uploader.py demo was failing with ValueError when trying to upload files
because @gen.coroutine decorated functions return tornado.concurrent.Future objects,
but asyncio.run() expects native coroutines.

This change converts the @gen.coroutine decorated functions to native async/await
syntax, which is the modern recommended approach and resolves the compatibility
issue with asyncio.run().

Changes:
- Remove tornado.gen import
- Convert @gen.coroutine decorators to async def
- Convert all yield statements to await
- Maintains backward compatibility with existing Tornado versions