It depends on the Pygments syntax highlighting library: http://pygments.org/

There are the following configuration options:

pygments_style = <style> (default: colorful)
highlightfiles = <fileset> (default: size('<5M'))
highlightonlymatchfilename = <bool> (default False)

highlightonlymatchfilename will only highlight files if their type could be identified by their filename. When this is not enabled (the default), Pygments will try very hard to identify the file type from content and any match (even matches with a low confidence score) will be used.