oci: Resolve manifests by digest like done for blobs.

Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>

content/oci/readonlyoci.go

@@ -32,6 +32,7 @@ import (
 	"oras.land/oras-go/v2/internal/fs/tarfs"
 	"oras.land/oras-go/v2/internal/graph"
 	"oras.land/oras-go/v2/internal/resolver"
+	"oras.land/oras-go/v2/internal/spec"
 )
 
 // ReadOnlyStore implements `oras.ReadonlyTarget`, and represents a read-only
@@ -186,6 +187,38 @@ func loadIndex(ctx context.Context, index *ocispec.Index, fetcher content.Fetche
 	return nil
 }
 
+// getMediaType returns the media type for the given path.
+// By default, descriptor.DefaultMediaType is returned.
+func getMediaType(fsys fs.FS, path string) string {
+	file, err := fsys.Open(path)
+	if err != nil {
+		return descriptor.DefaultMediaType
+	}
+
+	content, err := io.ReadAll(file)
+	if err != nil {
+		return descriptor.DefaultMediaType
+	}
+
+	manifest := &ocispec.Manifest{}
+	err = json.Unmarshal(content, manifest)
+	if err == nil {
+		return ocispec.MediaTypeImageManifest
+	}
+
+	// WARNING This code may never be called as an artifact can be unmarshalled
+	// to a manifest.
+	// This leads to test failures like:
+	// readonlyoci_test.go:350: ReadOnlyStore.Resolve() = {application/vnd.oci.image.manifest.v1+json sha256:063351176d525cd595e4741dbf2cbed7eb449534c7afc6ad9ae926da4ef24660 354 [] map[] [] <nil> }, want {application/vnd.oci.artifact.manifest.v1+json sha256:063351176d525cd595e4741dbf2cbed7eb449534c7afc6ad9ae926da4ef24660 354 [] map[] [] <nil> }
+	artifact := &spec.Artifact{}
+	err = json.Unmarshal(content, artifact)
+	if err == nil {
+		return spec.MediaTypeArtifactManifest
+	}
+
+	return descriptor.DefaultMediaType
+}
+
 // resolveBlob returns a descriptor describing the blob identified by dgst.
 func resolveBlob(fsys fs.FS, dgst string) (ocispec.Descriptor, error) {
 	path, err := blobPath(digest.Digest(dgst))
@@ -204,7 +237,7 @@ func resolveBlob(fsys fs.FS, dgst string) (ocispec.Descriptor, error) {
 	}
 
 	return ocispec.Descriptor{
-		MediaType: descriptor.DefaultMediaType,
+		MediaType: getMediaType(fsys, path),
 		Size:      fi.Size(),
 		Digest:    digest.Digest(dgst),
 	}, nil