Case insensitive Check for file extension

This line 

        if ( ! in_array(end(explode('.', $filename)), $this->_allowedExt) ) {

fails if files are named .JPG which can happen a lot on windows with files straight from a camera etc.  Changed to this

        if ( ! in_array(end(explode('.', strtolower($filename))), $this->_allowedExt) ) {

to avoid raising error.