| A | Library file (Unix) |
| A | Object code library |
| A3L | Authorware 3.x library |
| A3M | Authorware MacIntosh file (unpackaged) |
| A3W | Authorware Windows file (unpackaged) |
| A4L | Authorware 4.x library |
| A4M | Authorware MacIntosh file (unpackaged) |
| A4P | Authorware file (packaged without runtime) |
| A5L | Authorware 5.x library |
| A5W | Authorware Windows file (unpackaged) |
| AA | Audible audio file (commonly used for downloadable audio books) |
| AAM | Authorware shocked file |
| AAS | Authorware shocked packet |
| AB | Applix Builder file |
| ABF | Adobe Binary Screen Font |
| ABK | Corel Draw AutoBackup |
| ABK | Backup file (PrintMaster Gold) |
| ABM | Audio album file (HitPlayer) |
| ABO | Applix Builder Turbo file |
| ABP | Barcode profile file in Alver Valley Software's AVS Barcode Source |
| ABS | Standard GNU compiler output file for a PC platform |
| ABS | Sometimes used to denote an abstract (as in an abstract or summary of a scientific paper) AutoBackup |
| ABS | MPEG Audio Sound file |
| AC$ | AutoCAD Undo Info file |
| AC3 | Dolby Digital audio files used on DVDs |
| ACA | HTTP animation file (Microsoft Agent) |
| ACA | Project Manager Workbench file |
| ACB | ACBM Graphic image |
| ACC | DR-DOS Viewmax file |
| ACD | Character definiton file (Microsoft Agent) |
| ACE | ACE Archiver Compression file |
| ACF | HTTP character file (Microsoft Agent) |
| ACI | ACI development appraisal (ACIWEB) |
| ACL | Corel Draw 6 keyboard accelerator file |
| ACM | Interplay compressed sound file (Fallout 1,2, Baulder's Gate) |
| ACM | Dynamic Link Library (DLL) |
| ACM | Windows system directory file |
| ACP | Microsoft Office Assistant Preview file |
| ACR | American College of Radiology file |
| ACS | Character structered storage file (Microsoft Agent) |
| ACT | FoxPro Foxdoc Action Diagram |
| ACT | Action Presentation |
| ACT | Documenting wizard action diagram (Microsoft Visual FoxPro) |
| ACT | Microsoft Office Assistant Actor file |
| ACV | Used to Compress and decompress audio data |
| AD | After Dark screensaver |
| Ada | Ada source text file (non-GNAT) |
| ADB | HP 100LX Organizer Appointment database |
| ADB | Ada source text body file (GNAT) |
| ADC | Scanstudio 16 color Bitmap Graphic file |
| ADD | OS/2 adapter driver file used in the boot process |
| ADF | Amiga disk file |
| ADI | AutoCAD device-independent binary plotter file |
| ADL | QEMM Mca adaptor description library |
| ADM | Windows NT policy template |
| ADM | After Dark MultiModule screensaver (Microsoft) |
| ADN | Lotus 1-2-3 Add-In file |
| ADP | FaxWorks Faxmodem setup file |
| ADP | Astound Dynamite file |
| ADP | Dynamic Page file (AOLserver) |
| ADR | Smart Address address book |
| ADR | After Dark Randomizer screensaver |
| ADS | Ada source text specification file (GNAT) |
| ADT | AdTech Fax file |
| ADT | HP NewWave datafile for card applications |
| ADX | Lotus Approach dBase Index |
| ADX | Archetype Designer Document |
| ADX | Dynazip Active Delivery script |
| ADZ | Packed ADF file (Extracts with WinZip) |
| AE | Author/Editor file (SoftQuad) |
| AEP | ArcExplorer project file |
| AF2 | ABC FlowCharter 2.0 Flowchart |
| AF3 | ABC Flowchart |
| AFC | Apple Sound file |
| AFI | Truevision Bitmap graphic |
| AFM | HP NewWave Cardfile application |
| AFM | Adobe metrics |
| AG | Applix graphic |
| AI | Corel Trace drawing |
| AI | Adobe Illustrator drawing |
| AIF | Audio Interchange File, a sound format used by Silicon Graphics and Macintosh applications |
| AIFC | Similar to AIF (compressed) |
| AIFF | Similar to AIF |
| AIM | AOL Instant Messenger Launch file |
| AIN | AIN Compressed archive |
| AIO | APL transfer file |
| AIS | Velvet Studio Instruments file |
| AIS | Xerox Arry of Intensity Samples Graphic |
| AIS | ACDSee Image Sequence |
| AIX | HP NewWave Cardfile Application data |
| AKB | HaleyAuthority Authority Knowledge Base File |
| AKW | Contains all A-keywords in the RoboHELP Help Project Index Designer not associated with topics |
| ALAW | European Telephony audio |
| ALB | JASC Image Commander album |
| ALI | Document file (SAP proprietary format) |
| ALIAS | Alias Image |
| ALL | WordPerfect for Windows General printer information file |
| ALL | Arts & Letters Library |
| ALS | Alias Image |
| ALT | WordPerfect Library Menu |
| AM | Applix SHELF Macro |
| AMF | DSMIA/Asylum module music (Crusader,No Remorse,Aladdin) |
| AMF | Music file (Advanced Module Format) |
| AMG | ACTOR System image |
| AMG | AMGC Compressed Archive File |
| AMI | Annotation file (Cocreate SolidDesigner) |
| AMS | Extreme's Tracker Module |
| AMS | Velvet Studio music module (MOD) File |
| AMU | Sony Photo Album |
| AN | Text file (Sterling Software) (Groundworks COOL Business Team Model) |
| ANC | Canon Computer Pattern Maker file that is a selectable list of pattern colors |
| ANI | Microsoft Windows Animated cursor |
| ANM | DeluxPaint Animation |
| ANN | Windows 3.x Help annotation |
| ANS | ANSI Text file |
| ANT | SimAnt for Windows saved game |
| AOM | Adobe Download Manager Online Manager Shortcut |
| AOM | Adobe After Effects Output Settings |
| AOS | Nokia 9000 Add-on software |
| AOT | Applicatio binary object template file (ZenWorks snAPPshot) |
| AP | WHAP Compressed Amiga archive |
| AP | Applix Presents file |
| APC | Lotus 1-2-3 Printer driver |
| APC | Compiled application file (Centura Team Developer) |
| APD | Lotus 1-2-3 Printer driver |
| APD | Dynamic application library file (Centura Team Developer) |
| APE | Winamp Plugins AVS File |
| APE | Monkey's Audio Lossless Audio Compression Format |
| APF | Project file (Allaire) (Created by Homesite) |
| APF | Lotus 1-2-3 Printer driver |
| API | Lotus 1-2-3 Printer driver |
| API | Application Program Interface; used by Adobe Acrobat |
| APL | APL Workspace file |
| APL | Application library file (Centura Team Developer) |
| APP | Symphony Add-in Application |
| APP | Generated application or active document (Microsoft Visual FoxPro) |
| APP | dBase Application Generator Object |
| APP | DR-DOS Executable Application |
| APP | FoxPro Generated Application |
| APP | Normal mode application file (Centura Team Developer) |
| APR | Employee Appraiser Performance Review file |
| APR | Lotus Approach 97 View file |
| APR | ArcView project file |
| APS | Advanced patching systems with error checking, (Similar to IPS) |
| APS | Microsoft Visual C++ file |
| APT | Text mode application file (Centura Team Developer) |
| APT | Lotus Approach Data view file |
| APX | Borland C++ Appexpert database |
| APX | Lotus Approach Paradox-Specific information file |
| AQ | Applix data |
| ARC | SQUASH Compressed archive |
| ARC | LH ARC (old version) compressed archive |
| ARF | Automatic Response file |
| ARI | Aristotle audio file |
| ARI | ARI Compressed archive |
| ARJ | Robert Jung ARJ compressed archive (ARJ) |
| ARK | ARC File Archiver CPM/Port archive |
| ARL | AOL v4.0 organizer file |
| ARR | Atari Cubase Arrangement |
| ART | Canon Crayola art |
| ART | Clip Art |
| ART | Ray Tracer file |
| ART | First Publisher Raster graphic |
| ART | AOL Image file compressed using the Johson-Grace compression algorithm |
| ART | Xara Studio drawing |
| ARX | AutoCAD Runtime Extension |
| ARX | ARX Compressed Archive |
| AS | Applix Spreadsheet |
| ASA | Microsoft Visual InterDev file |
| ASC | PGP armored encrypted file |
| ASC | ASCII text file |
| ASD | Lotus 1-2-3 Screen driver |
| ASD | Microsoft Advanced Streaming Format (ASF) description file; opens with NSREX |
| ASD | Astound Presentation |
| ASD | WinWord AutoSave file |
| ASE | Velvet Studio Sample file |
| ASF | Microsoft Advanced Streaming Format |
| ASF | StratGraphics Datafile |
| ASF | Lotus 1-2-3 Screen font |
| ASF | Music file (Electronic Arts) |
| ASH | TASM 3.0 Assembly language header |
| ASI | Borland C++/Turbo C Assembler Include file |
| ASM | Pro/E assembly file |
| ASM | Assembler Language source file |
| ASO | Astound Dynamite Object |
| ASP | Procomm Plus setup and connection script |
| ASP | Astound Presentation |
| ASP | Active Server Page (an HTML file containing a Microsoft server-processed script) |
| AST | Claris Works "assistant" file |
| AST | Astound multimedia file |
| ASV | DataCAD Autosave file |
| ASX | Microsoft Advanced Streaming Redirector file |
| ASX | Video file |
| ASX | Cheyenne Backup script |
| AT2 | Aldus Persuasion 2.0 Auto Template |
| ATM | Adobe Type Manager data/info file |
| ATT | AT&T Group 4 bitmap |
| ATW | AnyTime Deluxe for Windows personal information manager file from Individual Software MultiModule screensaver |
| AU | Audio U-law (pronounced mu-law) |
| AU | Sun/NeXT/DEC/UNIX sound file |
| AUD | Audio file (Westwood Studios) (Kyrandia 3,C&C,RedAlert,C&C:TS) |
| AUP | Audacity project file |
| AUX | ChiWriter Auxilliary Dictionary file |
| AUX | TeX/LaTeX Auxilliary Reference file |
| AVA | Avagio Publication |
| AVB | Inculan Anti-Virus virus infected file |
| AVG | AVG Virus Information Database |
| AVI | Microsoft Audio Video Interleaved file for Windows movie |
| AVR | Audio Visual Research file |
| AVS | Stardent AVS-X Image |
| AVS | Application Visualization System file |
| AVX | File Extension (ArcView) |
| AW | Applix Words file |
| AW | HP AdvanceWrite Text file |
| AWD | FaxView Document image |
| AWE | Acrobat Bookmark XML File |
| AWK | AWK Script/Program |
| AWM | Animation Works Movie |
| AWR | Telsis file for digitally stored audio |
| AWS | StatGraphics Data file |
| AXL | ArcIMS XML project file |
| AXT | ASCII application object template (ZenWorks snAPPshot) |
| AXX | ARJ compressed file from a multi-volume archive (xx = a number from 01 to 99) |
