It is entirely up to the client system to decide what to
do with a document of a particular type. For example,
on my system at home, I have configured Netscape to
xpdf when given something of the
application/pdf. I could have used
acroread as well.
Or, I could have simply had it not do anything other than prompt
me to save the document to a file.
- The server cannot control what the client chooses to do with a
- The client generally trusts the server to provide a correct
content-type, and only looks that the file name for clues
if the server failed to provide any content-type at all.
This interaction between server and client, provider and consumer
strikes exactly the right balance, although it appears that some
recent  Email software has started to violate the the standards by
trying to pass the names of programs to be called, instead of
content-types when sending messages.