Don't overwrite download_content_type if already set #754

Job	Run time
check-dev-build	4s
sign-assets / test-release / build-debug / build	1m 24s
sign-assets / smoke-inflator / build-release / build	1m 45s
sign-assets / test-release / test-build	1m 0s
sign-assets / smoke-inflator / smoke-test-inflator	50s
sign-assets / test-release / notify	0s
sign-assets / smoke-inflator / notify	0s
sign-assets / sign	2m 41s
upload-release-s3	11s
upload-rpm	27s
upload-deb	31s
upload-dmg	28s
upload-inflator	2m 47s
upload-metadata-tester	1m 28s
notify-discord / Notify Discord	5s
	13m 41s

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