Fix upload_from_file with "application/json"

[oittaa]

Fixes #161

[codecov-commenter]

Codecov Report

Merging #162 (de0a856) into main (b0a6d35) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   88.00%   88.01%   +0.01%     
==========================================
  Files           7        7              
  Lines         875      876       +1     
  Branches      121      122       +1     
==========================================
+ Hits          770      771       +1     
  Misses         77       77              
  Partials       28       28              

Flag	Coverage Δ
unittests	88.01% <100.00%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
gcp_storage_emulator/server.py	85.87% <100.00%> (ø)
gcp_storage_emulator/storage.py	94.17% <100.00%> (+0.03%)	⬆️

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