Billy Zha

Technically, a valid artifact reference can also be a valid file path. When a user only provides one argument, it's hard to which of the below usages matches the real...

IMHO, it's better to follow the practice suggested by [CLI Guideline](https://clig.dev/#arguments-and-flags) and make file reference specified by flags. E.g. The usage of `oras attach` should changed to ```bash oras attach...

@bedla plus don't forget to [fix DCO check failure](https://github.com/oras-project/oras/pull/1713/checks?check_run_id=41229226968).

This should also resolve part of #1404

Sample error output (phrasing and wording still TBD) ```console > ./bin/linux/amd64/oras attach --artifact-type test/example "a-------" Error: "a-------": invalid reference: missing registry or repository Have you specified an artifact reference to...

BTW this bug generates unwanted referrer tags (256-xxx) ONLY in registries that haven't implemented [OCI Distribution Spec v1.1.0](https://github.com/opencontainers/distribution-spec/blob/v1.1.0/spec.md).

@njucjc Can you help confirm if this has met your expectation described in #1435? You may build from main branch or use `ghcr.io/oras-project/oras:main`. Thanks in adavnce.

Closing as it's resolved.