method getStringAt

This method returns the string at the given index and format. This method will return null if data is not found.

function onDragOver()
{
    var event = shell.currentEvent;
    if (event.sourceData && event.sourceData.hasFormat("application/x-file"))
    {
        var path = event.sourceData.getStringAt(0, "application/x-file");
        if (path)
        {
            // Do something.
        }
    }
}