MIME's "Content-Disposition" Header

Recently I have seen the following header used:
Content-Disposition: attachment; filename="abc.txt"  .
The format of this header is not defined in the Sept. 1993 definition
of MIME, so where is  this header defined, or what is its
current accepted usage.

Thanks in advance,
Mark K. Joseph
Wollongong Advanced Applications Center
Santa Cruz, CA.