In BinaryFilesResponse.php's "prepare" method, the partial response is created only if no "If-Range" request header found, or if it matches the response's ETag.
Default behavior of this response is to not send ETag header, but Last-Modified, so we have no browser cache.
As per the spec, "If-Range" request header value may also be compared with the response's Last-Modified time.
And that would be usefull :-)