Some of my URLs look like this:
[external links are visible to admins only]
--> This is a html file generated by Zenphoto and not a jpg picture! All those URLs will not be crawled, if I add the jpg extension under "Exclude from sitemap extension". It seems that the sitemap generator checks only if the string ".jpg" exists somewhere in the URL. This is wrong. It should check if an URL ends with that extension. Due to this behaviour I do not get a valid sitemap, which contains all html files, but no pictures. Hope this will be fixed soon.