Today, I found out that my sitemap.xml file got problem. It showed me this error message when I tried to view it.
Error loading stylesheet: An XSLT stylesheet does not have an XML mimetype
I think the problem come from the recently Google Sitemap Generator plugin upgrade.
To solve this kind of problem, open up your .htaccess and add this line at the end of the file.
AddType application/xml .xsl
Save and upload.