CaseResult/index.jelly includes summary.jelly, but a change made by 0460a04f1cafe580d7fd8dff79b28d1cd440346a for
JENKINS-19884 makes summary.jelly override the content-type header.
If the servlet container haven't yet committed a response, this will change the effective content type to "text/plain" while some other servlet containers will no doubt render this as "text/html".
I think summary.jelly will send out "text/html" content type, unless I'm missing something. If it really needs to be "text/plain", it should only do so when it's loaded from XmlHttpRequest.