(Sitemap) XML files are being served by webserver as type 'text/html'

[rrdein]

I am using Sitemap XML to generate sitemaps for my zencart site on a subdomain (shared hosting). I am able to access the sitemaps through my browser and the type reads, 'text/html'. When I try to upload this through Google Webmaster tools I get an error saying that the file is an HTML file. I have tried modifying the MIME type in the .htaccess in my /sitemap folder, and it has not worked, but I may have done it wrong.

All DNS requests for this domain are directed to Godaddy, and Godaddy forwards it transparently to mydomain.com on another host.

I have used Sitemap XML successfully with my original domain, but I need to generate new sitemap files that use the new domain address instead of the old domain. I don't know how to do this when they are all being served as 'text/html', and I don't know how to tell my server to serve XML in a way that is acceptable to Google. Any help is appreciated.