I am working with an older version of coldbox, but I perceive that the issue still exists in the current version. Event Caching fails to provide a content type if the request is not binary. I made two changes to my local copy of coldbox to resolve this issue. My changes were in Coldbox.cfm but I think that was renamed to Bootstrap.cfm.
The code below is a reference point. Note that the code calls writeOutput but does not set the content type.