One sitemap for all content types ( From Official Google Webmaster central blog)
Have you ever wanted to submit your various content types (video, images, etc.) in one Sitemap? Now you can! If your site contains videos, images, mobile URLs, code or geo information, you can now create—and submit—a Sitemap with all the information.
Site owners have been leveraging Sitemaps to let Google know about their sites’ content since Sitemaps were first introduced in 2005. 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. Here's an example of a Sitemap that contains a reference to a standard web page for Web search, image content for Image search and a video reference to be included in Video search:
Source: Official Google Webmaster central blog http://googlewebmastercentral.blogspot.com/2010/06/sitemaps-one-file-many-content-types.html