The Media Viewer is a software feature basically copied from the Lightbox used by Flickr. Its development and rollout by WMF has been object to controversies. For more information, see:

If you want to globally disable Media Viewer you need to create a page called "global.js" and insert the following code: mw.config.set("wgMediaViewerOnClick", false);.1

You can also disable this feature in your global preferences: select the "Appearance" section and uncheck "Enable Media Viewer".

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