Add content type for Canon CR3 raw files
Canon has introduced a new RAW image format CR3 which is based on the iso mp4 container. currently this is either seen as a generic mp4 file or applicaiton/octet-stream.
It's basically a mp4 container with ftyp crx. A more detailed analysis of the file structure is available at https://github.com/lclevy/canon_cr3/blob/master/readme.md