Add file streaming section to the streaming documentation page

docs/sanic/streaming.md

@@ -119,3 +119,25 @@ async def index(request):
 ```
 
 If a client supports HTTP/1.1, Sanic will use [chunked transfer encoding](https://en.wikipedia.org/wiki/Chunked_transfer_encoding); you can explicitly enable or disable it using `chunked` option of the `stream` function.
+
+## File Streaming
+
+Sanic provides `sanic.response.file_stream` function that is useful when you want to send a large file. It returns a `StreamingHTTPResponse` object and will use chunked transfer encoding by default; for this reason Sanic doesn't add `Content-Length` HTTP header in the response. If you want to use this header, you can disable chunked transfer encoding and add it manually:
+
+```python
+from aiofiles import os as async_os
+from sanic.response import file_stream
+
+@app.route("/")
+async def index(request):
+    file_path = "/srv/www/whatever.png"
+
+    file_stat = await async_os.stat(file_path)
+    headers = {"Content-Length": str(file_stat.st_size)}
+
+    return await file_stream(
+        file_path,
+        headers=headers,
+        chunked=False,
+    )
+```