/**
* Extracts the image size and WebP type from a file based on the chunk list
* @param array $chunks Chunks as extracted by RiffExtractor
+ * @param string $filename
* @return array Header data array with entries 'compression', 'width' and 'height', where
* 'compression' can be 'lossy', 'lossless', 'animated' or 'unknown'
*/