Set content type of release assets to zip

[JohnnyMorganz]

Fixes #496

[codecov]

Codecov Report

Merging #510 (71d2dd8) into master (7672adf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #510   +/-   ##
=======================================
  Coverage   97.66%   97.66%           
=======================================
  Files          14       14           
  Lines        5346     5346           
=======================================
  Hits         5221     5221           
  Misses        125      125           

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