Core: short circuit protocol detection for S3

[bpandola]

We can't rely on the ContentType header because it's often set based on the MIME type of the object data being uploaded.

Closes #10010
Supersedes and Closes #10011

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.20%. Comparing base (665e817) to head (a11da25).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10012   +/-   ##
=======================================
  Coverage   93.20%   93.20%           
=======================================
  Files        1328     1328           
  Lines      120734   120736    +2     
=======================================
+ Hits       112535   112537    +2     
  Misses       8199     8199           

Flag	Coverage Δ
servertests	28.92% <100.00%> (+<0.01%)	⬆️
unittests	93.18% <100.00%> (+<0.01%)	⬆️

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[martindurant]

Thank you!