E-mail, postings, and web files can be labelled with a specific Media Type (or MIME type) for use by a MIME-capable program. The two Media Types for use in sci.math.research (other than "text/plain", which is can be used anywhere) are "application/x-tex" and "application/x-latex". See the welcome message for details about posting TeX to sci.math.research.
However, many web files, including web pages, are labelled by suffixes rather than Media Types. For this reason Netscape also uses a file that converts suffixes to Media Types. There is also a global and local version of this file; the local version is called ".media.types" by default. You need both a Media Types file and a Mailcap file to make Netscape as useful as it was intended to be for accessing preprint servers and reading math papers on the Web in the general.