This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
File name extensions tell you what sort of file you are dealing with so that you will know which program(s) to use to unpack, unShrink, display, etc. the file. Many programs which create such files do not automatically add an extension-- for example, most of the disk images on the Golden Orchard CD are Diskcopy files with no name extension. Many other programs which create files suggest a default extension as part of the name-- GS-ShrinkIt generally suggests ".SHK"-- but, the user can change this and save under any legal name desired. (One popular change is using ".SDK" for ShrinkIt whole-disk archive files.) Some extensions indicate a filetype recognized by Apple II ProDOS; but, often, the extension is just for user information or to help some utility recognize the file as one it can deal with. For such files the actual ProDOS filetype is usually TXT, BIN, or SHK. What is it? (What program do I use?) ------------------------------------------------------------ .2MG also .2IMG- XGS IIgs disk image file usually 800k or larger (GS ASIMOV2; PC Imgutnew.exe) .AAF Apple Archive Format [TEXT] for source code (aaf.unpacker) .ACU NuFX Applelink archive (ShrinkIt) .ALU usually a multi-file, non-compressed A2 archive (ALU) .APF GS super-res "Apple Preferred" packed graphics format (Platinum Paint, Convert 3200, etc.) .ARC PC Archive (GS-ShrinkIt or DeArc2E or PC Arc program) .BMP Windows Bit-Mapped graphics format (GS Convert 3200; many PC viewers) .BNX NuFX with BLU header. (ShrinkIt) .BNY BLU archive. (ShrinkIt) .BQY NuFX with BLU header. (ShrinkIt) .BSC BinScii file. [TEXT] (BinScii or GScii) .BSE A GSHK .SEA file with a Binary II header (ShrinkIt) .BSQ BinScii'd NuFX file. [TEXT] (BinScii plus ShrinkIt on the result) .BXY NuFX archive with a Binary II header. (ShrinkIt) .CPT Compactor Pro archive (Compactor Pro on a Mac only) .DIMG Diskcopy disk image file usually produced by a Mac (GS Clone or Diskcopy; Mac Diskcopy; PC Imgutnew.exe) .DO a .DSK file specified as having data in "DOS 3.3 Order" i.e. uses DOS 3.3 sector ordering (A2 DSK2FILE and GS ASIMOV) .DSK standard emulator disk image-- length is 143360 bytes for 5.25" disk images (A2 DSK2FILE and GS ASIMOV) .EXE A2 Executioner file [TEXT]. (On A2; some files may EXEC properly under only DOS 3.3.) .GIF Graphics Interchange Format: Compressed picture (IIGIF for //e; Super Convert, ... on GS; PC, etc.: many viewers and editors) .GZ GZip PC archive format often used for storing A2 emulator disk images (GS GZPK v2 plus PMPUnZIP or Angel; PC WinZIP) .HDV Raw (DSK) ProDOS ordered disk image file 800K or greater in size; used by emus as a virtual hard disk (ASIMOV2 on IIgs) .HQX Mac BinHex file. [TEXT] (BinHex on Mac or GScii) .HTM HTML [TEXT] with embedded Text commands (Web browsers, web editors, etc.) .IMAGE Diskcopy images (see .DIMG) .IMG Type IMG or "user #7" Copy II Plus disk image file (A2 Copy II Plus v6.x or v7.x) .IMG is sometimes used for Diskcopy images (see .DIMG) .JPG PC JPEG hi-res, hi-color graphics format (GS JPEG.VIEWER, etc. B/W only or PC, Unix viewers) .LBR a multi-file, non-compressed A2 archive (Librarian) .LHA LHA Archive (PC/Amiga LZH program) .LZH LZH Archive (PC/Amiga LZH program) .NIB emulator disk image (typical length: 232960) for protected 5.25" software (A2 Saltine's Super Transcopy) .PCX PC graphics format (GS Convert 3200; PC many viewers) .PD compressed GS multi-palette graphics file w/o palettes (GS SuperPac) .PNG PC PING hi-res, hi-color graphics format (PC viewer) .PO a .DSK file specified as having data in "ProDOS Order" i.e. uses ProDOS sector ordering (A2 DSK2FILE and GS ASIMOV) .PS compressed GS multi-palette graphics file with palettes (GS SuperPac) .QQ BLU archive. (ShrinkIt) .SDK ShrinkIt disk image, usually NuFX-compressed (ShrinkIt) .SEA Self-extracting A2 ShrinkIt or Mac ShrinkIt archive (depending upon kind, run on Apple IIgs or Mac) .SHK usually an A2 NuFX-compressed archive; non-A2-compatible Mac .SHK archives also exist (GS ShrinkIt* / Mac unshrinker utility / PC Nulib-- does not extract GS resource forks) .SIT Mac StuffIt archive. (Stuffit on Mac or GS ShrinkIt) GS-ShrinkIt will not decode StuffIt Deluxe files. .TAR Unix Tape Archive (Unix tar with -xvf option, GS EXE tar) .TGZ Gzipped .TAR file .uu Unix uuencode file [TEXT] (//e uudecode, Unix uudecode) .uue Unix uuencode file [TEXT] (//e uudecode, Unix uudecode) .TIFF Graphics format (GS SHR Convert) .TXT [TEXT] An ASCII text file (Text editors, word processors, etc.) .UU Unix uuencode file [TEXT] (A2 uudecode or Unix uudecode) .UUE Unix uuencode file [TEXT] (A2 uudecode or Unix uudecode) .Z Compressed file (GS-ShrinkIt or Unix uncompress) .ZIP PC Zip Archive (GS PMPUnZIP or UNZIP [GS Shell EXE] or PC WinZIP, PKUNZIP, Unix unzip) .ZOO PC Zoo Archive (GS-ShrinkIt??? or PC ZOO program) * Note: GS-ShrinkIt (= GSHK) can handle all ShrinkIt files except .SDK (shrunken disk) files of 5.25" DOS 3.3 disks created by 8-bit ShrinkIt. 8-bit ShrinkIt does not work for GS files having a resource fork or GS .SEA files. ---------------------------- By: Apple's ftp site ... Most files are in one of a few common formats, and many are a combination. .sit StuffIt 1.5.1 archives .hqx BinHex 4.0 file .bin Binary file .image Diskcopy 4.2 image file .txt plain ASCII text file .bsc Apple II BinSCII file .shk Apple II ShrinkIt file Most of the Macintosh files are BinHexed StuffIt files. This means you need to transfer the file, then read the license agreement which is prepended to it (with any text processor), use BinHex or any utility which can read BinHex 4.0 files to decode the BinHex to a StuffIt archive, then use UnStuffIt or the StuffIt Expander (or a similar utility) to decompress the .sit file into the final file. In some cases the final file is a .image file. These are exact duplicates of floppy disks (with verified checksums). Use Diskcopy to convert these files into floppy disks for installation. Some Apple System Software is in this format. Most of the Apple II files are either straight text or BinSCII'd ShrinkIt files. This means you need to transfer the file, then use BinSCII to convert the .bsc file to a ShrinkIt file, then use ShrinkIt to create the final file or disk. Note: Apple calls their BinSCII'd .SHK files ".bsc" instead of ".bsq". It is fairly common for uploaders and ftp sites to tag any BinSCII'd file as ".bsc". The rationale is that, once a user un-BinSCII's a file, he or she will find an .SHK, .ZIP, etc. file and know how to continue. By: David Kopper