RISC OS filetype
RISC OS filetype, or Acorn filetype, is a system of 12-bit file type identifiers used on RISC OS. It is essentially the RISC OS equivalent of a filename extension. A filetype is usually written as three hexadecimal characters, like "FF9".
A number of RISC OS archive formats have a 32-bit "load address" field. If the three high nibbles of this field are all 0xf, then the next three nibbles contain the filetype. Formats that have such a field include ArcFS, Spark, Squash, and ZIP (in extra field #0x4341).
Sometimes, when a RISC OS file is transferred to a foreign system, its filetype is appended to the filename, separated with a comma.