Fix HEIF file support

[naszta]

Summary

generic.heif should be saved as heif and not hif.

Context

Fixing downloaded file extension.

[AndreyNikiforov]

How did you get heif into iCloud? Specific device? What happens during download?

[naszta]

HEIF files skipped completely. Canon R6 Mark II has heif output. An example, if you would like to try. Its content type on iCloud is public.heif. Apple itself uses hif extension (I tested that), but heif is more widely used so I picked that one as an output.