On Windows, files served with backward slash in the name

If `upload_to` parameter of `FileField` is set, the `name` attribute of `File` model contains a slash. Since this is relative path on the filesystem, on Windows this is a backward slash (`\`). This may not be a problem, however, it is passed unchanged to `url()` and the resulting file URL contains a mix of forward and backward slashes. When using `URL_METHOD_2`, it is even escaped.