I can't get Media information for PNG file

Yes, there are limitations on this functionality; this media information is not available for all files/file types. The media_info field is an optional field, and when include_media_info is set to true in the /2/files/get_metadata request, media_info will be populated with the media information if/when it is available. The media information will not always be available though, due to a variety of factors, such as file type, size, and compatibility. I'll pass this along as a feature request to improve the availability of this, but I can't promise if or when that might be done.