Last modified: 2014-10-17 17:39:09 UTC
Due to bug 67056, MediaViewer fails to display thumbnails on beta, unless they have already been generated in production.
The underlying issue is that MediaViewer thumbnail guessing does not work well with frontend caches which cache 404s (see bug 64554 fore details). That's not a problem on WMF sites which have thumbnail handlers enabled, and not a problem on other sites which have thumbnail guessing disabled by default; Beta is the exception. We might want to just disable thumbnail guessing there; that would mean significantly different behavior on test and production, though.
*** Bug 72190 has been marked as a duplicate of this bug. ***