const nsIFilePicker = Components. interfaces. nsIFilePicker;
var fp = Components. classes ["@mozilla.org/filepicker;1"].
         createInstance (nsIFilePicker);
fp. init (window, "Open any file", nsIFilePicker. modeOpen);
fp. appendFilters(nsIFilePicker. filterXML);
if (fp. show () == nsIFilePicker. returnOK)
{
    ...
}