Any content that's cached by batcache gets re-sent with a text/html content-type header, regardless of the original header.
Note the header given here (a live site using Batcache):
http://collingswoodlib.org/index.php?bsuite_mycss=print
Despite the explicit text/css header being specified here:
http://plugins.trac.wordpress.org/browser/bsuite/trunk/bsuite.php#L963
I've confirmed this with other content served through Batcache, including things like this image:
http://collingswoodlib.org/files/2008/11/cropped-collingswood2.jpg