feat(storage): Store blob content-type in extended attributes

After the discussion in #116, this stores the blob content types
in extended attributes when using the filesystem backend.

If the underlying filesystem doesn't support extended attributes,
storing blobs won't work; also, if extended attributes get removed,
blobs won't be served anymore. We can relax this behavior if
needed (i.e. log errors but still accept to store or serve blobs).
However, since the Docker Engine (and possibly other container
engines) won't accept to pull images from a registry that doesn't
use correct content types for manifest files, it could be argued
that it's better to give a hard fail. (Otherwise, the container
engine gives cryptic error messages like "missing signature key".)

I can change that behavior (and log errors but still store/serve
blobs to the filesystem) if you think it's better.

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