Mime Types

The more documented reason for the existance of a Mime Types file is
to do file extension to Mime Type mapping. If this is the only reason
for this file, can anyone tell me why the Mime Types file (for Apache
for example) includes Mime Types with no file extensions?

Eg:

multipart/form-data
multipart/header-set
multipart/mixed
multipart/parallel
multipart/related
multipart/report
multipart/signed
application/vnd.uplanet.channel-wbxml
application/vnd.uplanet.list-wbxml
application/vnd.uplanet.listcmd-wbxml
application/vnd.wap.wbxml
application/xml


Or are these Mime Types being defined here for a valid reason? Is
there another way that the web server could determine the Mime Type of
content other than from the file extension?

Why are these entries required? Do they just exist for completness?

Thanks,
Wally