fix/Middleware: only swap out response stream if we are not serving a file
* previously would always write stream to own MemoryStream, but this is an issue when getting actual files since we must read it all in first, THEN find out we aren't responding with html and write our stream back to the response. --> caused weird download behaviour
* now take advantage of the other middleware and only swap out streams when we know we are doing a directory (letting Blazor respond).