Legacy (v1 to v3) artifact uploads stored the request's
`Content-Encoding` header in the column that v4 artifacts read as their
`Content-Type`.
- Store only the `gzip` marker on legacy uploads, the only encoding
their clients send
- Pick the PDF and audio/video serve headers only for content types that
`mime.ParseMediaType` accepts
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Co-authored-by: silverwind <me@silverwind.io>
Basavaraj S mandsilverwind
4f744d15be