GdThumb::determineFormat PHP Method

    protected function determineFormat()
    {
        if ($this->isDataStream === true) {
            $this->format = 'STRING';
            return;
        }
        $formatInfo = getimagesize($this->fileName);
        // non-image files will return false
        if ($formatInfo === false) {
            if ($this->remoteImage) {
                $this->triggerError('Could not determine format of remote image: ' . $this->fileName);
            } else {
                $this->triggerError('File is not a valid image: ' . $this->fileName);
            }
            // make sure we really stop execution
            return;
        }
        $mimeType = isset($formatInfo['mime']) ? $formatInfo['mime'] : null;
        switch ($mimeType) {
            case 'image/gif':
                $this->format = 'GIF';
                break;
            case 'image/jpeg':
                $this->format = 'JPG';
                break;
            case 'image/png':
                $this->format = 'PNG';
                break;
            default:
                $this->triggerError('Image format not supported: ' . $mimeType);
        }
    }