Add mimetype field to to packfile object

[nagem]

Fixes #931

Changes that require mimetype in jobs/add unearthed that the mimetype for a packfile upload was never persisted to the db.

Review Checklist

• Tests were added to cover all code changes
• Documentation was added / updated
• Code and tests follow standards in CONTRIBUTING.md

[codecov-io]

Codecov Report

Merging #932 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #932   +/-   ##
=======================================
  Coverage   90.19%   90.19%           
=======================================
  Files          48       48           
  Lines        6413     6413           
=======================================
  Hits         5784     5784           
  Misses        629      629

Flag	Coverage Δ
#python	90.19% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
api/placer.py	94.52% <ø> (ø)	⬆️

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[ryansanford]

Nice turnaround. I was able to confirm existing packfile uploads had the mimetype added, and new uploads now had it present.

I'm going to test against a larger database now before approving.

[kofalt]

Hah, another cgi_field haunt eh 👻

[ryansanford]

LGTM -- sanitized production DBs updated within a couple seconds with the expected result.