Idle thought...

It used to be that, when an image was clicked, it would generate a
pageview of a local File: page (for Commons images, usually a "dummy"
local page). On some projects, it would instead go to the
corresponding Commons file page.

However, the use of mediaviewer means that a significant fraction of
clicks on images will not lead to a new pageview; the user is
satisfied with the lightbox, closes it in place, and stays on the
page.

In the eswiki case, this seems to be a very convincing explanation for
the Commons drop. As a project which has not allowed local uploads for
a long time, I believe any image links went straight to Commons rather
than to the local File page used on enwiki.

Which leads to an obvious question - on other projects, such as enwiki
or frwiki, have we accounted for a drop in *file page* views?  What do
overall pageview numbers look like using, say, just mainspace/ns0?