When the Automatic button is selected, Fetch looks for standard filename suffixes in order to decide whether a file is a text file or a binary file. You can modify the list of recognized suffixes with the Suffix Mapping command. When the command is given a dialog box is displayed, showing all the current suffix mappings.
A suffix mapping associates a suffix (such as “.hqx”) with a file type (either Text, Binary, or BinHex) and optionally with type/creator codes. The “Text” type means that files with the associated suffix should be downloaded as text files; similarly, the “Binary” type means that the files should be downloaded as binary files. The BinHex type means that the file should be downloaded as text (since BinHex files are a kind of text file) but that non-BinHex material should be ignored. If a type/creator pair has been specified text or raw binary files will be given that type and creator. For instance, a file with the “.Z” suffix is usually given the type/creator pair of LZIV/ZIVM (the codes used by MacCompress), since “.Z” is the standard ending for compressed files and MacCompress is a program for reading such files.
Suffix mappings are only in force when the Automatic button is selected. And information in the file overrides the application specified in the suffix mapping: if a file is in BinHex, MacBinary, MacHost or AppleSingle format, it will include information about the application it belongs to.