Switching to webp for preview thumbnails

[Vankerkom]

WebP is baseline since 2020, it reduces bandwidth by 25% compared to jpg.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.91%. Comparing base (7238941) to head (82727f5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #372   +/-   ##
=======================================
  Coverage   92.91%   92.91%           
=======================================
  Files          14       14           
  Lines        1185     1185           
  Branches      228      228           
=======================================
  Hits         1101     1101           
  Misses         83       83           
  Partials        1        1           

Flag	Coverage Δ
unittests	92.91% <100.00%> (ø)

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[soruly]

Not all API users are using it on web browsers, they may be using it on native apps, or with 3rd party services like chat bots which has unknown media format support. So jpg is not a format I'd like to drop. webp is good, but supporting more than one media format makes things complicated. This is also true for video preview.