it is more or less the default dir for uploading images that is configured as [cdn path](http://sonata-project.org/bundles/media/master/doc/reference/installation.html#id1) for the media bundle. See also fe. [here](http://capifony.org/#4_setup_server) and [here](http://symfony.com/doc/2.0/cookbook/doctrine/file_uploads.html). Using configuration the allowed file types can be limited, see [here](https://github.com/sonata-project/SonataMediaBundle/blob/master/DependencyInjection/Configuration.php#L213)