The current webservice class does not follow redirects (default behaviour of QNetworkManager). In the case of cover art this would be particularly useful since images are often moved around servers, CDNs etc; but also for the main requests. See
PICARD-52 for some examples.
For simplicity I think initially it could ignore the complexity of POSTs that return redirects (and the http spec issues that tend to come with either matching browser behaviour or matching "official spec" behaviour for various 30x types).