Google "Your Sitemap appears to be an HTML page"

Hi,

Previously I used to generate an .xml file using a free online service and upload it via ftp. Now I have switched to Infinite SEO, so I deleted robots.txt and sitemap.xml from my directory using ftp, I installed and setup Infinite SEO, then went to Webmaster Tools and submitted my sitemap as mywebsite.com/sitemap.xml but I get an error message "Your Sitemap appears to be an HTML page."