| .A2 | Apple | Datatiedostot | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .A3 | Apple | Datatiedostot | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .A3L | Adobe Systems Incorporated | Datatiedostot | Authorware 3 Library | Adobe Authorware | Adobe Systems Incorporated |
| .A3M | Adobe Systems Incorporated | Datatiedostot | Authorware 3 Macintosh File | Adobe Authorware | Adobe Systems Incorporated |
| .A3W | Adobe Systems Incorporated | Datatiedostot | Authorware 3 Windows File | Adobe Authorware | Adobe Systems Incorporated |
| .A4L | Adobe Systems Incorporated | Datatiedostot | Authorware 4 Library | Adobe Authorware | Adobe Systems Incorporated |
| .A4M | Adobe Systems Incorporated | Datatiedostot | Adobe Authorware Unpackaged File | Adobe Authorware | Adobe Systems Incorporated |
| .A4P | Avanquest Software | Datatiedostot | A4Desk Pro Project File | Avanquest A4Desk Pro | Avanquest Software |
| .A4R | Adobe Systems Incorporated | Datatiedostot | Adobe Authorware Package Course Data | Adobe Authorware | Adobe Systems Incorporated |
| .A4W | Adobe Systems Incorporated | Datatiedostot | Authorware 4 Windows File | Adobe Authorware | Adobe Systems Incorporated |
| .A5L | Adobe Systems Incorporated | Datatiedostot | Authorware 5 Library | Adobe Authorware | Adobe Systems Incorporated |
| .A65 | Adobe Systems Incorporated | Datatiedostot | Authorware 6.5 File | Adobe Authorware | Adobe Systems Incorporated |
| .AAB | Adobe Systems Incorporated | Datatiedostot | Adobe Authorware Binary Data | Adobe Authorware | Adobe Systems Incorporated |
| .AAE | Apple | Datatiedostot | Sidecar Image Format File | Microsoft Notepad | Microsoft Corporation |
| .AAM | Adobe Systems Incorporated | Datatiedostot | Authorware Map File | Adobe Authorware | Adobe Systems Incorporated |
| .AAO | U.S Army | Datatiedostot | America's Army Map File | America's Army | U.S Army |
| .AAR | Apache Software Foundation | Datatiedostot | Axis Archive | Apache Axis2 | Apache Software Foundation |
| .AAS | Adobe Systems Incorporated | Datatiedostot | Adobe Authorware Shocked Packet File | Adobe Authorware | Adobe Systems Incorporated |
| .AB1 | Thomas Hall | Datatiedostot | DNA Electropherogram File | Genome Compiler | Genome Compiler Corporation |
| .AB2 | Broderbund | Datatiedostot | Print Shop Address Book File | The Print Shop | Broderbund |
| .AB3 | Ulead Systems | Datatiedostot | PhotoImpact 3 Album File | PhotoImpact | Corel |
| .AB4 | ENIAC Computing | Datatiedostot | AB4 Accounting & Business Data File | AB4 Accounting & Business | ENIAC Computing |
| .AB6 | SINTEF | Datatiedostot | ABStat Data | ABstat | Open Source |
| .ABB | Unknown Developer | Datatiedostot | AlphaBB Data | Unknown Software | Unknown Developer |
| .ABD | Interscape | Datatiedostot | Adventure Builder Data | Adventure Builder | Interscape |
| .ABI | Thomas Hall | Datatiedostot | DNA Chromatogram File | Heracle BioSoft DNA Baser Sequence Assembler | Heracle BioSoft |
| .ABL | Ubisoft Blue Byte | Datatiedostot | ANNO 2070 Scenario Data | ANNO 2070 | Ubisoft Blue Byte |
| .ABM | Unknown Developer | Datatiedostot | Music Album | Redbana Audition | Redbana |
| .ABP | Alver Valley Software Limited | Datatiedostot | AVS Barcode Profile | AVS Barcode Source | Alver Valley Software Limited |
| .ABT | Braille2000, LLC | Datatiedostot | Annotated Braille Text File | Braille 2000 | Braille2000, LLC |
| .ABU | ASUS | Datatiedostot | Asus App Backup File | ASUS Backup | ASUS |
| .ABY | AOL | Datatiedostot | AOL Address Book | AOL | AOL |
| .AC | GNU Operating System | Datatiedostot | Autoconf Script | GNU Autoconf | GNU Operating System |
| .AC2 | National Instruments | Datatiedostot | SPICE AC/frequency Domain Output | Spice | National Instruments |
| .AC7 | National Instruments | Datatiedostot | SPICE AC/Frequency Domain Output File | Spice | National Instruments |
| .ACC | FKJ Software | Datatiedostot | Graphics Accounts Data File | Graphic Accounts | FKJ Software |
| .ACCESSOR | Microsoft Corporation | Datatiedostot | Microsoft Visual Studio Accessor File | Microsoft Visual Studio | Microsoft Corporation |
| .ACD | Sony Creative Software | Datatiedostot | ACID Project File | ACID XPress | Sony |
| .ACF | Valve | Datatiedostot | Steam Application Cache File | Valve Steam | Valve |
| .ACG | Bolide Software | Datatiedostot | Audio Comparer Group File | Audio Comparer | Bolide Software |
| .ACH | Rockwell Automation Inc. | Datatiedostot | RSLogix 5 Project File | RSLogix 5 | Rockwell Automation Inc. |
| .ACI | ACI | Datatiedostot | ACI Development Appraisal | ACI Appraisal Software | ACI |
| .ACX | Atari Corporation | Datatiedostot | Atari ST Program | Gemulator | Open Source |
| .ACZ | BeyondApp Co., Ltd | Datatiedostot | AppCraft Project | AppCraft | BeyondApp Co., Ltd |
| .ADH | Audible | Datatiedostot | Audible Manager File | Audible | Audible |
| .ADI | Active@ Data Recovery Software | Datatiedostot | Active@ Disk Image Backup File | Active@ Disk Image | LSoft Technologies Inc. |
| .ADIUMEMOTICON | The Adium team | Datatiedostot | Adium Dock Icon File | Adium | Open Source |
| .ADIUMSERVICEICONS | Adam Iser | Datatiedostot | Adium Service Icons | Adium | Open Source |
| .ADIUMSTATUSICONS | Adam Iser | Datatiedostot | Adium Status Icon | Adium | Open Source |
| .ADM | Microsoft Corporation | Datatiedostot | Administrative Template File | Microsoft Group Policy Object Editor | Microsoft Corporation |
| .ADRESSES | Apple | Datatiedostot | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .ADT | Swiftpage, Inc. | Datatiedostot | ACT! Document Template | Swiftpage Act | Swiftpage, Inc. |
| .ADU | Addictive Software | Datatiedostot | Addict User Dictionary File | Addict | Addictive Software |
| .ADV | Unknown Developer | Datatiedostot | Audio Driver File | Dune | Cryo Interactive Entertainment |
| .ADX | CRI Middleware | Datatiedostot | ADX Audio File | FFmpeg | FFmpeg team |
| .AER | Adobe Systems Incorporated | Datatiedostot | Adobe Atmosphere World | Adobe Atmosphere | Adobe Systems Incorporated |
| .AES | Open Source | Datatiedostot | AES Crypt Encrypted File | Packetizer AES Crypt | Open Source |
| .AFF | Kingsoft | Datatiedostot | Spellcheck Dictionary Description File | Kingsoft Writer | Kingsoft |
| .AFG | James Hutton Institute | Datatiedostot | Tablet Assembly Data File | Tablet | James Hutton Institute |
| .AFL | Piotr Fusik | Datatiedostot | AFLI-editor Picture Image | Recoil | Piotr Fusik |
| .AFR | The FreeBSD Community | Datatiedostot | FreeBSD File | FreeBSD | The FreeBSD Community |
| .AFT | Ancestry.com Inc. | Datatiedostot | Family Tree Database | Software MacKiev Family Tree Maker | Ancestry.com Inc. |
| .AG | Lotus Software | Datatiedostot | Lotus Agenda File Format | Lotus Agenda | Lotus Software |
| .AG4 | Microsoft Corporation | Datatiedostot | Microsoft Access G4 Data | Microsoft Access | Microsoft Corporation |
| .AGD | AgileGraph | Datatiedostot | AgileGraph Data File | AgileGraph | AgileGraph |
| .AGF | ESRI | Datatiedostot | Atlas GIS Native Binary Geodataset | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .AGGR | Adobe Systems Incorporated | Datatiedostot | Adobe Captivate Aggregate File | Adobe Captivate | Adobe Systems Incorporated |
| .AGN | Psion PLC | Datatiedostot | Psion Series 3a Agenda File | Psion Series 3 | Psion PLC |
| .AGP | RL Vision | Datatiedostot | ArtGem Project File | ArtGem | RL Vision |
| .AGX | Open Source | Datatiedostot | AGT Game File | AGiliTy | Open Source |
| .AH | The FreeBSD Community | Datatiedostot | FreeBSD Installation Data | FreeBSD | The FreeBSD Community |
| .AHB | Cosby Clifton Computer Consultants Ltd | Datatiedostot | AIM High Briefcase Report File | AIM High | Cosby Clifton Computer Consultants Ltd |
| .AHD | Microsoft Corporation | Datatiedostot | Dynamics AX Online Help Data File | Microsoft Dynamics AX | Microsoft Corporation |
| .AHF | Anadigm | Datatiedostot | AnadigmDesigner ASCII Code | Anadigm Designer 2 | Anadigm |
| .AHI | Microsoft Corporation | Datatiedostot | Microsoft Dynamics AX Online Help Index | Microsoft Dynamics AX | Microsoft Corporation |
| .AHP | Autodesk, Inc. | Datatiedostot | AutoCAD Help Data | Autodesk AutoCAD | Autodesk, Inc. |
| .AI | Adobe Systems Incorporated | Datatiedostot | Adobe Illustrator File | Adobe Illustrator CC | Adobe Systems Incorporated |
| .AIA | MIT Media Lab | Datatiedostot | App Inventor Project | MIT App Inventor | MIT Media Lab |
| .AIF | Apple | Datatiedostot | Audio Interchange File Format | Microsoft Windows Media Player | Microsoft Corporation |
| .AIH | ESRI | Datatiedostot | Attribute Index | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .AII | Progress Software Corporation | Datatiedostot | Telerik Test Studio Test Definitions File | Telerik Test Studio | Progress Software Corporation |
| .AIL | Mystik Media | Datatiedostot | AutoImager File | AutoImager | Mystik Media |
| .AINSO | Unknown Developer | Datatiedostot | CRYSTALS Chemical Crystallogaphy File | CRYSTALS | Unknown Developer |
| .AIU | Caphyon | Datatiedostot | Advanced Installer Updates Configuration File | Advanced Installer | Caphyon |
| .AIV | EllaZ Systems | Datatiedostot | AIVault Data File | AlVault | EllaZ Systems |
| .AJG | AJC | Datatiedostot | AJC Grep Project | AJC Grep | AJC |
| .AKD | Aimsoft | Datatiedostot | AIM Keys File | AIM Keys | Aimsoft |
| .AKM | Srinivas Annam | Datatiedostot | Aksharamala Keymap Binary Data | Aksharamala | Interscape |
| .ALC | Ableton | Datatiedostot | Ableton Live Clip File | Ableton Live | Ableton |
| .ALD | Microsoft Corporation | Datatiedostot | Dynamics AX Application Label Data File | Microsoft Dynamics AX | Microsoft Corporation |
| .ALDRIN | Apple | Datatiedostot | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .ALF | Swiftpage, Inc. | Datatiedostot | ACT! Lookup File | Swiftpage Act | Swiftpage, Inc. |
| .ALG | Pascal Brachet | Datatiedostot | AlgoBox Algorithm File | AlgoBox | Pascal Brachet |
| .ALI | Microsoft Corporation | Datatiedostot | Dynamics AX Label Index File | Microsoft Dynamics AX | Microsoft Corporation |
| .ALM | Alpha Software | Datatiedostot | Alpha Five Library | Alpha Anywhere - Alpha Software | Alpha Software |
| .ALN | UlteraTech Group | Datatiedostot | LexNavigator Database Update | LexNavigator | UlteraTech Group |
| .ALR | Campbell Wild | Datatiedostot | ADRIFT Language Resource File | ADRIFT Generator | Campbell Wild |
| .ALT | Microsoft Corporation | Datatiedostot | Dynamics AX Temporary File | Microsoft Dynamics AX | Microsoft Corporation |
| .AMB | Bolide Software | Datatiedostot | All My Books Database File | All My Books | Bolide Software |
| .AMC | Pegasys | Datatiedostot | AMC Video File | Apple QuickTime Player | Apple |
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