[BUG] Preview not working *sometimes*

[aleksasiriski]

Original urls sometimes don't work, for various reasons, we should implement using thumbnail as fallback in case this happens.

Can this be done purely on frontend side? Maybe it would make sense to send both original and thumbnail url with hashes to proxy, and then proxy decides whether it works or not.

There should be an indicator on frontend to tell the user that the fallback is being used.