ResponseContentTypeVoter uses the extension of request if none set, it uses the Magnolia server default extension, so when using a delivery endpoint, it is not cached because in falls into:

/modules/cache/config/contentCaching/defaultPageCache/browserCachePolicy/policies/dontCachePages

resolves as html and sets the no cache headers.

code here:

String defaultExtension = ServerConfiguration.getInstance().getDefaultExtension();

in class info.magnolia.cms.beans.config.MIMEMapping

I guess fixing this issue will make many installations work in a different way

To reproduce just open this link https://demopublic.magnolia-cms.com/.rest/delivery/tours/v1

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Development decision :
Because ResponseContentTypeVoter was deprecated so we need to replace with RequestExtensionVoter which is a replacement for it (MGNLCACHE-290). And in order to make the class more configurable, we decided to add new flag named "emptyExtensionVote" to control this voter behavior in case of empty extension. Default value if not set is true (for backward compatibility). So in case of rest endpoint, if users want to cache it, they just need to add the flag to the dontCachePages voter and set its value to false.