python3.6
Using aiohttp, aiofiles library, asynchronous operation
The server code is as follows. After starting the service, it will listen to 0.0.0.0:8080 by default
No exception handling, applicable to small files, for reference only

file_server.py:

import aiofiles
import asyncio
import os
from aiohttp.web import Response
from aiohttp import web

def query_parse(req):
    obj = req.query_string
    queryitem = []
    if obj:
        query = req.query.items()
        for item in query:
            queryitem.append(item)
        return dict(queryitem)
    else:
        return None

async def download(request):
    query = query_parse(request)
    filename = query.get('file')
        # There is no exception handling. Only when the / tmp/files directory exists on the server and the requested file is in the directory, can it be downloaded normally
    file_dir = '/tmp/files'
    file_path = os.path.join(file_dir, filename)
    if os.path.exists(file_path):
        async with aiofiles.open(file_path, 'rb') as f:
            content = await f.read()
        if content:
            response = Response(
                content_type='application/octet-stream',
                headers={'Content-Disposition': 'attachment;filename={}'.format(filename)},
                body=content
                            )
            return response

        else:
            return
    else:
        return

loop = asyncio.get_event_loop()
app = web.Application(loop=loop)
app.router.add_route(method='get', path='/download', handler=download)
web.run_app(app)
loop.close()

python3 file_server.py running server

The client code is as follows:
download.py:

import aiohttp
import asyncio
import aiofiles

# Visit the download interface function of the server and download the file l5u.jpg under the specified directory (/ tmp/files /) on the server
url = 'http://localhost:8080/download'
params = {'file': 'l5u.jpg'}

async def download():
    async with aiohttp.ClientSession() as session:
        async with session.get(url, params=params) as resp:
            content = await resp.content.read()
        async with aiofiles.open('/tmp/test.jpg', 'wb') as f:
            await f.write(content)

loop = asyncio.get_event_loop()
loop.run_until_complete(download())

Execute python3 download.py to check whether / tmp/test.jpg exists and whether opening is normal
Or directly visit http://localhost:8080/download?file=l5u.jpg with a browser. The name of the downloaded file is the same as the name of the file requested to be sent (the name of the file existing on the server)