[MM-27277] Enable stream mode for file uploads (#6568)

* Enable stream mode for file uploads

* Add comment

actions/file_actions.jsx

@@ -17,9 +17,13 @@ export function uploadFile(file, name, channelId, rootId, clientId) {
  return request.
  post(Client4.getFilesRoute()).
  set(Client4.getOptions({method: 'post'}).headers).
- attach('files', file, name).
+
+ // The order here is important:
+ // keeping the channel_id/client_ids fields before the files contents
+ // allows the server to stream the uploads instead of loading them in memory.
  field('channel_id', channelId).
  field('client_ids', clientId).
+ attach('files', file, name).
  accept('application/json');
  };
 }