| .A | Open Source | Developer Files | Static Library | Microsoft Notepad | Microsoft Corporation |
| .A2W | Carnegie Mellon University | Developer Files | Alice World | Alice | Carnegie Mellon University |
| .A68 | Digital Equipment Corporation | Developer Files | PDP-10 Algol-68 Compiler Source | Unknown Software | Unknown Developer |
| .A86 | Eric Isaacson Software | Developer Files | A86 Assembly Language Source Code | A86/A386 | Eric Isaacson Software |
| .A8S | Steven Glanville | Developer Files | Anim8or Script | Anim8or | Steven Glanville |
| .ABA | Palm | Developer Files | Palm Address Book Archive | Palm Desktop | Palm |
| .ABC | Adobe Systems Incorporated | Developer Files | ActionScript Byte Code File | Adobe Flash Player | Adobe Systems Incorporated |
| .ABL | Ubisoft Blue Byte | Developer Files | ANNO 2070 Scenario Data | ANNO 2070 | Ubisoft Blue Byte |
| .ABO | Pierre Chambart, Jean-Christophe FILLIATRE and Sylvain Le Gall | Developer Files | Applix Builder File | Camomile (Library File) | Pierre Chambart, Jean-Christophe FILLIATRE and Sylvain Le Gall |
| .ACD | Sony Creative Software | Developer Files | ACID Project File | ACID XPress | Sony |
| .ACH | Rockwell Automation Inc. | Developer Files | RSLogix 5 Project File | RSLogix 5 | Rockwell Automation Inc. |
| .ACP | Alfresco Software, Inc. | Developer Files | Alfresco Content Package | Alfresco | Alfresco Software, Inc. |
| .ACTIONSCRIPT | Adobe Systems Incorporated | Developer Files | Adobe Flash ActionScript Data | Adobe Animate CC 2017 | Adobe Systems Incorporated |
| .ACU | Micro Focus International plc | Developer Files | Machine Independent COBOL Object File | ACUCOBOL-GT | Micro Focus International plc |
| .AD2 | Unknown Developer | Developer Files | ADPCM 2-bit Compressed Voice | Awave Studio | FMJ-Software |
| .ADA | Open Source | Developer Files | Ada Source Code | Notepad++ | Don Ho |
| .ADB | Open Source | Developer Files | Ada Body File | MacroMates TextMate | MacroMates |
| .ADIUMSCRIPTS | Adam Iser | Developer Files | Adium AppleScripts File | Adium | Open Source |
| .ADS | Open Source | Developer Files | Ada Specification File | MacroMates TextMate | MacroMates |
| .AEM | Planit Solutions, Inc. | Developer Files | Alphacam Wire EDM Parameter Macro | Planit Alphacam | Planit Solutions, Inc. |
| .AGC | John Bridges | Developer Files | AfterGRASP Compiled Script | AfterGRASP | Open Source |
| .AGI | Digium Inc. | Developer Files | Asterisk Gateway Interface File | Asterisk | Digium Inc. |
| .AGO | Soft Machines | Developer Files | Autolog Script | AutoLog | Soft Machines |
| .AGP | RL Vision | Developer Files | ArtGem Project File | ArtGem | RL Vision |
| .AGS | John Bridges | Developer Files | AfterGRASP Script File | AfterGRASP | Open Source |
| .AIML | Richard S. Wallace | Developer Files | Artificial Intelligence Markup Language | Notepad++ | Don Ho |
| .AIO | IBM | Developer Files | APL Transfer Format File | IBM APL | IBM |
| .AJB | Planit Solutions, Inc. | Developer Files | Alphacam Water Jet VB Macro | Microsoft Visual Basic | Microsoft Corporation |
| .AJM | Planit Solutions, Inc. | Developer Files | Alphacam Water Jet Parameter Macro | Planit Alphacam | Planit Solutions, Inc. |
| .AKP | Propellerhead | Developer Files | Akai Sampler File | ReCycle | Propellerhead |
| .ALB | Alpha Software | Developer Files | Alpha Five Library | Alpha Anywhere - Alpha Software | Alpha Software |
| .ALM | Alpha Software | Developer Files | Alpha Five Library | Alpha Anywhere - Alpha Software | Alpha Software |
| .ALP | AnyLogic | Developer Files | AnyLogic Project File | AnyLogic | AnyLogic |
| .ALX | BlackBerry | Developer Files | BlackBerry Application Loader XML File | BlackBerry Desktop Software | BlackBerry |
| .AM | GNU Operating System | Developer Files | Automake Makefile Template | Automake | GNU Operating System |
| .AM3 | Indigo Rose | Developer Files | AutoPlay Media Studio 3.0 Project | AutoPlay Media Studio | Indigo Rose |
| .AM4 | Indigo Rose | Developer Files | AutoPlay Media Studio 4 Project File | AutoPlay Media Studio | Indigo Rose |
| .AM5 | Indigo Rose | Developer Files | AutoPlay Media Studio 5 Project File | AutoPlay Media Studio | Indigo Rose |
| .AM6 | Indigo Rose | Developer Files | AutoPlay Media Studio 6 Project | AutoPlay Media Studio | Indigo Rose |
| .AML | Intel Corporation | Developer Files | ACPI Machine Language File | DSDTSE | otRock |
| .AMW | Anark Corporation | Developer Files | Anark Media Studio Workspace | Anark Studio | Anark Corporation |
| .ANDROIDPROJ | Microsoft Corporation | Developer Files | Microsoft Visual Studio Android Project | Microsoft Visual Studio | Microsoft Corporation |
| .ANM | Unknown Developer | Developer Files | 3D Animation File | CRYENGINE | Crytek |
| .APA | Rockwell Automation Inc. | Developer Files | RSView Development Project Archive | Rockwell Automation RSView Studio or RSView ME | Rockwell Automation Inc. |
| .APB | Planit Solutions, Inc. | Developer Files | Alphacam Punch VB Macro | Planit Alphacam | Planit Solutions, Inc. |
| .APM | Adobe Systems Incorporated | Developer Files | Aldus Placeable Metafile Image | GIMP | The GIMP Development Team |
| .APPX | Microsoft Corporation | Developer Files | Windows 8 App Package | Microsoft Windows 8 | Microsoft Corporation |
| .APPXMANIFEST | Microsoft Corporation | Developer Files | Windows Store App Source Manifest | Microsoft Visual Studio | Microsoft Corporation |
| .APR | ESRI | Developer Files | ArcView Project File | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .APS | Microsoft Corporation | Developer Files | Visual C++ Resource File | Microsoft Visual C++ | Microsoft Corporation |
| .AP_ | Google | Developer Files | ADT Android Developer Package | 7-Zip | Igor Pavlov |
| .ARQ | BMC | Developer Files | BSM Action Request System Client Macro | BMC | BMC |
| .ARR | Lemkesoft | Developer Files | Amber Graphic File | Lemkesoft GraphicConverter | Lemkesoft |
| .ART | Autodesk, Inc. | Developer Files | ArtCAM Model | ArtCAM | Autodesk, Inc. |
| .ARY | Hewlett-Packard | Developer Files | SmartStart Scripting Toolkit Data | SmartStart | Hewlett-Packard |
| .AS | Adobe Systems Incorporated | Developer Files | Adobe ActionScript File | Microsoft Notepad | Microsoft Corporation |
| .AS3 | IBM | Developer Files | APL*PLUS Source Code File | IBM APL | IBM |
| .ASC | Symantec Corporation | Developer Files | PGP (Pretty Good Privacy) ASCII Armored File | Mozilla Thunderbird | Mozilla |
| .ASI | Borland | Developer Files | ASsembler Include | Asi Loader | Interscape |
| .ASM | Open Source | Developer Files | Assembly Language Source Code File | Microsoft Notepad | Microsoft Corporation |
| .ASO | Adobe Systems Incorporated | Developer Files | ActionScript Object | Adobe Creative Cloud | Adobe Systems Incorporated |
| .ASX | Microsoft Corporation | Developer Files | Microsoft ASF Redirector File | VLC Media Player | VideoLAN |
| .ASZ | Open Source | Developer Files | AlphaControls Skin Package | AlphaControls | Open Source |
| .ATG | Unknown Developer | Developer Files | Netmino File | Netmino | Unknown Developer |
| .ATP | Planit Solutions, Inc. | Developer Files | Alphacam Lathe Post File | Planit Alphacam | Planit Solutions, Inc. |
| .AU3 | Jonathan Bennett | Developer Files | AutoIt V3 Script | AutoIt | Jonathan Bennett |
| .AUT | Jonathan Bennett | Developer Files | AutoIt Script File | AutoIt | Jonathan Bennett |
| .AWK | Free Software Foundation | Developer Files | AWK Script | Apple TextEdit | Apple |
| .AWL | Lavasoft | Developer Files | Ad-Aware Language Data | Ad-Aware | Lavasoft |
| .AXB | Alpha Software | Developer Files | Alpha Five Xbasic Script | Alpha Five | Alpha Software |
| .AXE | Microsoft Corporation | Developer Files | AutoRoute Map File | Microsoft AutoRoute | Microsoft Corporation |
| .AXF | ARM Ltd | Developer Files | ARM RealView Compiler Object | RealView Development Suite | ARM Ltd |
| .AXS | Microsoft Corporation | Developer Files | ActiveX Script File | Microsoft Notepad | Microsoft Corporation |
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