In the OCI image specification, the MediaType field is reserved
and otherwise undefined; assume that manifests without a media
in storage are OCI images or image indexes, and determine which
by looking at what fields are in the JSON. We do keep a check
that when unmarshalling an OCI image or image index, if it has
a MediaType field, it must match that media type of the upload.
Signed-off-by: Owen W. Taylor <otaylor@fishsoup.net>
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