Small improvement docs: better example for file uploads.

docs/web.rst

@@ -743,15 +743,20 @@ should use :meth:`Request.multipart` which returns a :ref:`multipart reader
 
         # /!\ Don't forget to validate your inputs /!\
 
-        mp3 = await reader.next()
+        # reader.next() will `yield` the fields of your form
 
-        filename = mp3.filename
+        field = await reader.next()
+        assert field.name == 'name'
+        name = await field.read(decode=True)
 
+        field = await reader.next()
+        assert field.name == 'mp3'
+        filename = field.filename
         # You cannot rely on Content-Length if transfer is chunked.
         size = 0
         with open(os.path.join('/spool/yarrr-media/mp3/', filename), 'wb') as f:
             while True:
-                chunk = await mp3.read_chunk()  # 8192 bytes by default.
+                chunk = await field.read_chunk()  # 8192 bytes by default.
                 if not chunk:
                     break
                 size += len(chunk)
@@ -789,10 +794,10 @@ with the peer::
         print('websocket connection closed')
 
         return ws
-    
-The handler should be registered as HTTP GET processor::    
-    
-    app.router.add_get('/ws', websocket_handler)    
+
+The handler should be registered as HTTP GET processor::
+
+    app.router.add_get('/ws', websocket_handler)
 
 .. _aiohttp-web-websocket-read-same-task: