Group-user.js

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/* Any JavaScript here will be loaded for registered users only */

/* Clean-up the file extension for new uploads and fix mobile */
$(function() {
	if (mw.config.get('wgCanonicalSpecialPageName') === 'Upload' && !$('#wpForReUpload').length && !$('.warning').length) {
		if (mw.config.get('wgMFMode')) {
			$("#wpUploadFile").change(function() {
				var ogFileName = $("#wpUploadFile").val().split('\\').pop();
				$('#wpDestFile').val(ogFileName);
			});
			$("#wpUploadFileURL").change(function() {
				var ogFileName = $("#wpUploadFileURL").val().split('/').pop();
				$('#wpDestFile').val(ogFileName);
			});
		}
		$('input[name=wpUpload]').click(function() {
			var file = $('#wpDestFile').val();
			if (file.includes('.')) {
				var fileName = file.substr(0, file.lastIndexOf('.')).replace(/\.+$/, '');
				var fileExt = file.split('.').pop().toLowerCase();
				if (fileExt == 'jpeg')
					fileExt = 'jpg';
				$('#wpDestFile').val(fileName + '.' + fileExt);
			}
		});
	}
});

MediaWiki:Group-user.js