method hasFormat

This method returns true if the transfer data object contains the given format.

function onPaste()
{
    var clipboard = shell.clipboard;
    if (clipboard)
    {
      if (clipboard.hasFormat("PNG"))
      {
        var png = clipboard.getStreamAt(0, "PNG");
        if (png)
        {
            // Do something.
        }
      }
    }
}