test: add asserts for mime types

[cristianmtr]

Part of #2479

[codecov]

Codecov Report

Merging #2531 (eb0ca8e) into master (60ee474) will increase coverage by 0.36%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2531      +/-   ##
==========================================
+ Coverage   85.61%   85.98%   +0.36%     
==========================================
  Files         152      152              
  Lines        9462     9462              
==========================================
+ Hits         8101     8136      +35     
+ Misses       1361     1326      -35     

Flag	Coverage Δ
daemon	46.82% <66.66%> (ø)
jina	85.96% <83.33%> (+0.39%)	⬆️

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Impacted Files	Coverage Δ
jina/flow/builder.py	96.25% <83.33%> (ø)
jina/flow/base.py	90.59% <0.00%> (+0.91%)	⬆️
jina/peapods/runtimes/zmq/zed.py	93.80% <0.00%> (+0.95%)	⬆️
jina/peapods/pods/compound.py	89.47% <0.00%> (+21.80%)	⬆️

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[maximilianwerk]

LGTM