MultimediaViewer offers a set of download size options, set in mmv.ui.utils.js
These are currently hardcoded/not configurable
These are impacted by $wgThumbnailSteps, so a user asking for a "small 640x480" download will in fact get the 960px thumb, which can be confusing.
Make these sizes configurable and throw a configuration error, if smaller sizes than provided in wgThumbnailSteps are used (not sure how to make this understandable to users yet).