File types indicate the kind of content a file contains and are
necessary for the operating system and applications to know how to deal with any given file. There are standard file types such as an ASCII Text file which would be indicated by the file type "TEXT" on a Mac and the file name extension ".TXT" on a PC.

On Macintosh file types are four letter codes, and are normally viewable only using a utility like ResEdit. However, the icon of a file is used to visually indicate its type:

structure 'BAS5'
compiled structure 'BASY'
data file 'dat5'
data segment 'dax5'
log file '4LOG'
4D Client Resource File 'res '
4D Client Local file 'LOC4'
4D Mac Plug-in '4DPX'

On Windows, file types are three letters appended to the filename following a period. See Tech Tip 00-256 from March of 2000 for details on how to view these file extensions

structure .4DB / .RSR
compiled structure .4DC / .RSR
data file .4DD / .4DR
data segment .4DS
log file .4DL
4D Client Resource File .RES
4D Client Local file .REX
4D Mac Plug-in .4DX / .RSR
4D Win Plug-in .4DX

For more information see the 4D online documentation page on this subject.

Also Chapter 3 of the Customizer Plus Reference (download Customizer Plus reference in PDF format.)