var myFiles = selectFiles(true, "jsx, theme, someOtherExtension");

function selectFiles(multiSelect, extensionList) {
    var infoMessage = multiSelect ? "Please select multiple files" : "Please select file";
    var filter = ($.os.indexOf("Windows") != -1) 
        ? "*." + extensionList.replace(/ /g, "").replace(/,/g, ";*.")
        : function (file) {
            var re = new RegExp("\.\(" + extensionList.replace(/,/g, "|").replace(/ /g, "") + ")$","i");
            if (file.name.match(re) || file.constructor.name === "Folder")
                return true
        };
    return File.openDialog(infoMessage, filter, multiSelect);
}