Backport #39855 by @basavaraj-sm05
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
Co-authored-by: Basavaraj S m <basavaraj@digiscrypt.com>
Co-authored-by: silverwind <me@silverwind.io>