Allow that content may be something other than a UnixFS file

[cdata]

Currently the spec requires that content be a Cid<IpfsUnixFsFile>. However, it isn't very clearly spelled out why it must be this format (though one can speculate about various IPFS ecosystem compatibility/coherence virtues).

Noting the availability of a metadata field, it should be possible to include a header to signal:

• The content is a UnixFS file, or..
• The content is something other than a UnixFS file.

By observing the metadata, a renderer of the file system can decide what it does or does not wish to render (based on support or other criteria).

Would it be possible to carve out an allowance for non-UnixFS files in the spec, or else perhaps shore up the requirement for UnixFS files with some supporting errata in the spec text?