fix attach opts

Signed-off-by: Sylvia Lei <lixlei@microsoft.com>

cmd/oras/attach.go

@@ -92,6 +92,7 @@ Example - Attach file to the manifest tagged 'v1' in an OCI layout folder 'layou
 	cmd.Flags().StringVarP(&opts.artifactType, "artifact-type", "", "", "artifact type")
 	cmd.Flags().IntVarP(&opts.concurrency, "concurrency", "", 5, "concurrency level")
 	cmd.MarkFlagRequired("artifact-type")
+	opts.EnableDistributionSpecFlag()
 	option.ApplyFlags(&opts, cmd.Flags())
 	return cmd
 }

cmd/oras/internal/option/target.go

@@ -17,7 +17,6 @@ package option
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"os"
 	"strings"
@@ -84,9 +83,6 @@ func (opts *Target) Parse() error {
 	switch {
 	case opts.isOCILayout:
 		opts.Type = TargetTypeOCILayout
-		if opts.Remote.distributionSpec.referrersAPI != nil {
-			return errors.New("cannot enforce referrers API for image layout target")
-		}
 		return nil
 	default:
 		opts.Type = TargetTypeRemote