Google now lets you submit various content types in one sitemap. For example, if you want to submit videos, images, mobile URLs, etc. in the same sitemap, you can do so.
"Site owners have been leveraging Sitemaps to let Google know about their sites’ content since Sitemaps were first introduced in 2005," explains Google Webmaster Trends Analyst Jonathan Simon. "Since that time additional specialized Sitemap formats have been introduced to better accommodate video, images, mobile, code or geographic content. With the increasing number of specialized formats, we’d like to make it easier for you by supporting Sitemaps that can include multiple content types in the same file."
"The structure of a Sitemap with multiple content types is similar to a standard Sitemap, with the additional ability to contain URLs referencing different content types," he adds. You can see an example here.
Google reminds webmasters that previous sitemap rules still apply. There is a 50,000 URL per file limit, and a 10MB uncompressed file size limit.