Your Sitemap appears to be an HTML page. Please use a suppo

Hi,
I’m using Google XML Sitemaps plugin, my sitemap :
http://www.3rabgamers.com/sitemap.xml
When I submit it in webmaster tools, it makes error especially in this sitemap :
http://www.3rabgamers.com/sitemap-pt-download-2014-08.xml
Error :Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
NOTE: I’m using WP Super Cache plugin, I have read that there is a setting in cache plugin may solve the problem but I don;t know how ?
Thanks..

https://wordpress.org/plugins/google-sitemap-generator/