This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
I. Ground, TFFE, and most other Apple II ftp sites These sites prefer .SHK files for stuff intended to run or be accessed on an Apple II. For example, a game which includes a program file, text Readme file, and folder of pic files would be shrinked into an .SHK file. It is best to Shrink even compressed picture, small binary, and icon files and upload them as .SHK files because everyone is used to dealing with .SHK files and the filetype of shrinked files is preserved. ProDOS diskettes should, usually, be uploaded as a collection of files in a regular .SHK file. This uses less space than a whole-disk archive file. DOS 3.3 diskettes should be uploaded as whole-disk archive .SDK files. (That is, you have an .SHK whole-disk archive but you change its name to end with ".SDK".) Pictures and diagrams you want to be both useable on an Apple II and viewable on-line should be converted to .GIF form (e.g. via Super Convert) and uploaded in this form. If you have several pictures or diagrams you want to be accessible off- line on an Apple II as well as a PC, Mac, etc., you can convert them to .GIF form, place them all in a .ZIP file on your PC, and upload the .ZIP file. All of the above would be uploaded in "binary" mode. Text which is intended to be readable on-line should be uploaded as plain Text in "ASCII" or "Text" mode or, on most sites today, in binary mode. Whenever you upload a game, utility, etc. to an ftp site, it's a good idea to also upload a brief Text file with a description of the uploaded item. For example, after uploading NARFGAME2.SHK (in binary mode), you could upload a brief description in a Text file named NARFGAME.TXT (in Text or binary mode). II. Asimov and other Apple II emulator ftp sites Upload files will, generally, be individual DOS 3.3 or ProDOS disk image (.DSK) files created on an Apple II by DSK2FILE or ASIMOV. Upload in binary mode. III. Comp.binaries.apple2 Programs, etc. posted to this newsgroup are normally .SHK files which have been Binscii'd-- i.e. after Shrinking, the file is changed to Text form via a binscii utility. IV. Comp.sources.apple2 Source files posted to this newsgroup are normally plain Text. By: Brian Hammack