fix(storage): respect ContentType in Client::UploadFile

[coryan]

Fixes #7520

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This change is 

[google-cloud-cpp-bot]

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For commit: 23d855880ba74d041193295d1f87e03a7686c1ab

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[codecov]

Codecov Report

Merging #7521 (23d8558) into main (38959b4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #7521   +/-   ##
=======================================
  Coverage   93.66%   93.67%           
=======================================
  Files        1361     1361           
  Lines      118101   118146   +45     
=======================================
+ Hits       110622   110668   +46     
+ Misses       7479     7478    -1     

Impacted Files	Coverage Δ
google/cloud/storage/internal/curl_client.cc	98.28% <100.00%> (+0.04%)	⬆️
...loud/storage/tests/object_file_integration_test.cc	100.00% <100.00%> (ø)
...bigtable/examples/bigtable_hello_instance_admin.cc	81.31% <0.00%> (-2.20%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.00% <0.00%> (-0.60%)	⬇️
google/cloud/pubsub/samples/samples.cc	92.25% <0.00%> (+0.23%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (+0.24%)	⬆️

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