Fix a sync error with manifests without mediaType

[git-hyagi]

fixes: #1746

[lubosmj]

Looks reasonable! Do you mind backporting this to 2.20 and releasing it?

[ipanova]

The root cause of this issue is not because an old manifest was synced. But because the manifest.json file did not contain at all mediaType field. In reality you look at raw manifest(that fails sync) you will see that this is a normal OCI schema2 image manifest.

ipanova@puffy:/tmp$ skopeo inspect docker://quay.io/fedora/fedora-bootc@sha256:54ac58d3b89726f3050d650d47d6d7f20a6b524d739a9bc03f4d84d23f7a2b09 --raw |jq .mediaType
null
ipanova@puffy:/tmp$ skopeo inspect docker://quay.io/fedora/fedora-bootc@sha256:66f42c1cd2ab599ee0f53e940f8c2f05baa78bd6a2179a2f66a034749da8ed61 --raw |jq .mediaType
null
ipanova@puffy:/tmp$ skopeo inspect docker://quay.io/fedora/fedora-bootc@sha256:66f42c1cd2ab599ee0f53e940f8c2f05baa78bd6a2179a2f66a034749da8ed61 --raw |jq .config
{
  "digest": "sha256:2c63889a84890052ebf642498e8b01c86ab4a832e89870fd52d2ff221920a4d9",
  "mediaType": "application/vnd.oci.image.config.v1+json",
  "size": 10958
}
ipanova@puffy:/tmp$ 

If you read the specs you will see that mediaType field is not required.

The changelog message needs to be re-written.

[patchback]

Backport to 2.20: 💚 backport PR created

✅ Backport PR branch: patchback/backports/2.20/d9ee026fa71ea1b6538b25ccaf63f120c88fd273/pr-1747

Backported as #1748

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