I'm getting the same error and one workaround I've found that works is to rename the stylesheet documents to have an xml extension. document.xsl becomes something like document-xsl.xml.
If you managed to come up with a better solution I'd love to hear it because I'm having to manually modify the files generated by third party software. I'd prefer it if I could just configure JBoss or Firefox to load them correctly.
I've changed the stylesheet to have .xml extension. But the result is still the same. It works fine on IE, but on Mozilla Firefox, it says:
"Error loading stylesheet: An XSLT stylesheet does not have an XML mimetype:"
I don't understand why it doesn't work, since I have configured jboss to recognize mime type .xsl and also .xml.