If this doesn't sound very useful by itself, you're right. However, this turns out to be all you need in order to add some useful functionality to mailcap-oriented programs such as mailto(1). For example, the following line in a mailcap file allows mailto to help the user include PostScript in outgoing mail:
application/postscript ; lpr -Pipa %s ; description="Include a Postscript File";compose="getfilename Postscript %s"
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