Ensure flexibility for unspecified media types

[jzelinskie]

This is a task for auditing the spec to make sure that nothing in the distribution specification prevents unspecified media types from being used for descriptors.

Related: opencontainers/image-spec#759

[vbatts]

The corresponding PR is merged. What is needed here?

[jzelinskie]

I need to skim over the spec to see if anything is blocking this. I think @jdolitsky and @stevvooe need to take a look as well.

We might want to clarify interactions around registries electing to reject specific content types on PUT /v2/<name>/manifests/<reference>. Maybe there should be an explicit error code for it, otherwise we're stuck using invalid manifest: https://github.com/opencontainers/distribution-spec/blob/master/spec.md#put-manifest

[jzelinskie]

I think we can close this since the only change that had to be made was in image-spec and now we have the artifact repository.