Add dot to extensions of added MIME types

[hugovk]

There's a suggestion in python/cpython#75223 / python/cpython#2895 to raise a ValueError when adding MIME type extensions with a dot.

This PR adds the dots.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.31%. Comparing base (f9211e3) to head (c4080f7).
Report is 58 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #622   +/-   ##
=======================================
  Coverage   82.31%   82.31%           
=======================================
  Files          72       72           
  Lines        7431     7431           
=======================================
  Hits         6117     6117           
  Misses       1314     1314           

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[bsipocz]

Thank you for the PR, it looks uncontroversial, so I go ahead with a merge.