Convert all content types to string #813

Job	Run time
unit-tests (ubuntu-latest, nightly)	39s
unit-tests (windows-latest, v0.11.1)	1m 21s
unit-tests (macos-latest, v0.11.1)	55s
unit-tests (windows-latest, nightly)	1m 0s
unit-tests (macos-latest, nightly)	1m 2s
unit-tests (ubuntu-latest, v0.11.1)	1m 12s
	6m 9s

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