Without a "/" in the MIME type, the content returned can be sniffed per <http://tools.ietf.org/html/draft-abarth-mime-sniff-04>, and because it starts out with a <script> tag, is likely to be sniffed as "text/html".
See Bug 37358 for the original fix and test case.
Created attachment 54765 [details]
Yes, David it is a typo. Sorry about that.
Committed r58604: <http://trac.webkit.org/changeset/58604>