Associates helper applications with particular file types

The mailcap file is used by the metamail program. Metamail reads the
mailcap file to determine how it should display non-text or multimedia
material. Basically, mailcap associates a particular type of file
with a particular program that a mail agent or other program can call
in order to handle the file.

Mailcap should be installed to allow certain programs to be able to
handle non-text files.