Google Code-in

See https://phabricator.wikimedia.org/T113697 and https://www.mediawiki.org/wiki/Extension:UploadWizard

The message "mwe-upwiz-multi-file-select" in UploadWizard says: "Hold down the Ctrl/Command key to make multiple non-adjacent selections".

It would be nicer to turn the key name into a parameter and fill it in runtime by identifying the user's operating system.

This can be done with 'jquery.client', $.client.profile().platform will give the information. It's already used in the "VisualEditor" extension for platform-specific keyboard shortcuts - see the code there as an example.