change logo_format type from string to image blob

[elfkuzco]

Rationale

See openzim/zimfarm#1572

Changes

• change logo_format to image blob

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.28%. Comparing base (a944214) to head (7d37539).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files           4        4           
  Lines         376      376           
  Branches       37       37           
=======================================
  Hits          347      347           
  Misses         25       25           
  Partials        4        4           

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[benoit74]

LGTM