Unlike the customary file type identification of 3 letters used on Atari or DOS systems, Apple computers classify files through a long integer number that corresponds to a sequence of 4 ASCII characters. This file identification is not a constituent of the name but is managed by the operating system internally. Since the file type is not visible to the user, it can also consist of cryptic character strings, which are meaningful only to the programmer.
Which file type is given to the files is set up in the programs by the programmer (e.g., with OPEN) and can be determined with the procedure Set_File Type from the Extension Library. However, file types created by the programmer have to include at least one capital letter because file types consisting of nothing but small letters and special characters were reserved for Apple's own specific, internal purposes.
The Omikron Basic editor applies the file type "OBAS" to tokenized BASIC code, "OLIB" is given to libraries, and all others are classified as "TEXT." The following table gives an overview of the most important file types predefined by Apple.
|'DFIL'||File for saving of accessories|
|'FFIL'||File for saving of fonts|
|'adev'||Network extension (e.g., EtherTalk 2.0)|
|'appe'||Program executable only in the background|
|'edtp'||Extension for the common use of graphical data|
|'edts'||Extension for the common use of sound-oriented data|
|'edtt'||Extension for the common use of text-oriented data|
|'ifil'||Resource collection for document systems|
|'pref'||File with default values|
|'qery'||Query document for data base accesses|
|'scri'||System extension for document systems|
|'tfil'||True type font|
|'ttro'||Teach text for read-only file|
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