Hi Daniel,

you should avoid to deal with static sitemap files as much as possible. They are used only if you have large sitemaps that need to be exported or cached as a file. The name assigned to the exported sitemaps is given according to the format, xml, images, etc so there is a reason for that.

What you want to do, is rewriting the sitemap link using the htaccess, to have:
http://www.mydomain.com/index.php?option=com_jmap&view=sitemap&format=xml

rewritten as:
http://www.mydomain.com/sitemap.xml

Please refer to this FAQ http://storejextensions.org/faq/how-can-i-rewrite-a-dynamic-xml-sitemap-url-to-sitemap-xml.html

PS: rewriting the sitemap link makes no difference for search engines