This article is from the Mac communications FAQ, by Bruce L Grubb BruceG6069@aol.com with numerous contributions by others.
File mapping is the method by which a file's application is identified
by a three or four letter extension. This feature was used by Internet
Config (MacOS 7.x) and later Apple's own Internet control panel to give
a downloaded file a "type" and "creator" by extension. The programs
ICFileDiverter and ICTypeChanger used this extension file mapping
to change "type" and "creator" to whatever is set by either Internet
Config or the Internet CP.
If something happens to file mapping or the file lacks an extension then
non-mac files cannot be given a "type" and "creator" and one must either
trust the MacOS to figure it out or try to determine what broad type
the file is (see [2.8]).