| .A2M | TallStick Software | Settings Files | TS-AudioToMIDI Settings File | TS-AudioToMIDI | TallStick Software |
| .AAP | Heikki Ylinen | Settings Files | Apollo Advanced Playlist | Apollo | ApolloDB |
| .AAWDEF | Lavasoft | Settings Files | Ad-Aware Definitions File | Ad-Aware | Lavasoft |
| .ABS | ComponentAce | Settings Files | Absolute Database File | ComponentAce Absolute Database | ComponentAce |
| .ACB | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Color Book File | Adobe Illustrator CC | Adobe Systems Incorporated |
| .ACF | Valve | Settings Files | Steam Application Cache File | Valve Steam | Valve |
| .ACO | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Color Swatch File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .ACS4 | Artem Izmaylov | Settings Files | AIMP Skin File | AIMP | Artem Izmaylov |
| .ACV | Adobe Systems Incorporated | Settings Files | Photoshop Curves File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .ACW | Microsoft Corporation | Settings Files | Windows Accessibility Wizard File | Microsoft Windows Accessibility Wizard | Microsoft Corporation |
| .ADIUMMENUBARICONS | Adam Iser | Settings Files | Adium Menu Bar Icon File | Adium | Open Source |
| .ADIUMMESSAGESTYLE | Adam Iser | Settings Files | Adium Message Style Settings | Adium | Open Source |
| .ADIUMSOUNDSET | The Adium team | Settings Files | Adium Sound Set File | Adium | Open Source |
| .ADM | Microsoft Corporation | Settings Files | Administrative Template File | Microsoft Group Policy Object Editor | Microsoft Corporation |
| .ADO | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Duotone Options File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .AES | Open Source | Settings Files | AES Crypt Encrypted File | Packetizer AES Crypt | Open Source |
| .AFW | iGrafx | Settings Files | Micrografx FlowCharter Work Area Data | iGrafx Designer | Corel |
| .AGDB | ANSYS, Inc. | Settings Files | ANSYS DesignSpace Data | ANSYS DesignSpace | ANSYS, Inc. |
| .AGG | New World Computing | Settings Files | Bulk Data File | Heroes of Might & Magic III | Ubisoft |
| .AHL | Open Source | Settings Files | eMule Metadata | eMule | Open Source |
| .AHS | Adobe Systems Incorporated | Settings Files | Adobe Halftone Screen File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .AHU | Adobe Systems Incorporated | Settings Files | Adobe Photoshop HSL File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .AIP | Adobe Systems Incorporated | Settings Files | Illustrator Plug-in | Adobe Illustrator CC | Adobe Systems Incorporated |
| .AIT | Adobe Systems Incorporated | Settings Files | Adobe Illustrator Template | Adobe Illustrator CC | Adobe Systems Incorporated |
| .AIU | Caphyon | Settings Files | Advanced Installer Updates Configuration File | Advanced Installer | Caphyon |
| .ALL | Steinberg | Settings Files | Cubase Project | Steinberg Cubase | Steinberg |
| .ALV | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Levels File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .ALX | BlackBerry | Settings Files | BlackBerry Application Loader XML File | BlackBerry Desktop Software | BlackBerry |
| .AMP | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Curves Map File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .AMS | Adobe Systems Incorporated | Settings Files | Adobe Monitor Setup File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .ANC | Canon Inc. | Settings Files | Canon Computer Pattern Maker Color Pattern List | Canon Raw Codec | Canon Inc. |
| .ANV | Birch Grove Software, Inc. | Settings Files | ActivTrak Net View File | ActivTrak | Birch Grove Software, Inc. |
| .AOM | Adobe Systems Incorporated | Settings Files | After Effects Output Module | Adobe After Effects | Adobe Systems Incorporated |
| .APC | Savvius, Inc. | Settings Files | AiroPeek Wireless Trace Capture File | AiroPeek | Savvius, Inc. |
| .API | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Inks File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .APM | Adobe Systems Incorporated | Settings Files | Aldus Placeable Metafile Image | GIMP | The GIMP Development Team |
| .APPCONTENT-MS | Microsoft Corporation | Settings Files | Microsoft Windows Application Content | Microsoft Windows 8 | Microsoft Corporation |
| .APPLICATION | Microsoft Corporation | Settings Files | ClickOnce Deployment Manifest File | Microsoft .NET Framework | Microsoft Corporation |
| .APPREF-MS | Microsoft Corporation | Settings Files | Microsoft Application Reference File | Microsoft ClickOnce | Microsoft Corporation |
| .ARG | Autodesk, Inc. | Settings Files | AutoCAD Profile | Autodesk AutoCAD | Autodesk, Inc. |
| .ARL | AOL | Settings Files | AOL Organizer File | AOL Organizer | AOL |
| .ARP | ElcomSoft | Settings Files | Advanced RAR Password Recovery Setup | Advanced Archive Password Recovery | ElcomSoft |
| .ARS | Adobe Systems Incorporated | Settings Files | After Effects Render Settings File | Adobe After Effects | Adobe Systems Incorporated |
| .ASE | Adobe Systems Incorporated | Settings Files | Adobe Swatch Exchange File | Adobe Illustrator CC | Adobe Systems Incorporated |
| .ASK | askSam Systems | Settings Files | AskSam Database | askSam | Softonic Corporate |
| .ASL | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Style | Adobe Photoshop | Adobe Systems Incorporated |
| .ASP | Microsoft Corporation | Settings Files | Active Server Page | Mozilla Firefox | Mozilla |
| .AST | Apple | Settings Files | ClarisWorks Assistant File | Apple AppleWorks | Apple |
| .ASV | MakeMusic, Inc. | Settings Files | Finale Auto-save File | Finale | MakeMusic, Inc. |
| .ASW | ACD Systems | Settings Files | ACDSee Slideshow Wizard File | ACD Systems ACDSee | ACD Systems |
| .ASWCS | Avast Software | Settings Files | Avast! Compressed Skin File | Avast Antivirus | Avast Software |
| .ASWS | Avast Software | Settings Files | Avast! Skin File | Avast Antivirus | Avast Software |
| .ATC | Autodesk, Inc. | Settings Files | AutoCAD Tool Catalog File | Autodesk AutoCAD Civil 3D | Autodesk, Inc. |
| .ATF | Molecular Devices | Settings Files | Axon Text File | Microsoft Excel | Microsoft Corporation |
| .ATH | Dell Boomi | Settings Files | Alienware AlienFX Theme File | Microsoft XML Notepad | Microsoft Corporation |
| .ATN | Adobe Systems Incorporated | Settings Files | Photoshop Actions File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .AUTOCONF | The Open Group | Settings Files | Autoconfiguration File | Unix | AT&T Labs |
| .AUX | Interscape | Settings Files | LaTeX Auxiliary File | MiKTeX | Christian Schenk |
| .AVE | Avid Technology | Settings Files | Avid User File | Avid Media Composer | Avid Technology |
| .AVS | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Variations File | Adobe Photoshop | Adobe Systems Incorporated |
| .AWE | Adobe Systems Incorporated | Settings Files | Adobe Acrobat Bookmark XML Data | Adobe Creative Cloud | Adobe Systems Incorporated |
| .AWL | Lavasoft | Settings Files | Ad-Aware Language Data | Ad-Aware | Lavasoft |
| .AXC | Microsoft Corporation | Settings Files | Microsoft Dynamics AX Configuration | Microsoft Dynamics AX | Microsoft Corporation |
| .AXL | Wattle Software | Settings Files | XML File | MacroMates TextMate | MacroMates |
| .AXP | Canneverbe Limited | Settings Files | CDBurnerXP Audio Compilation File | Canneverbe CDBurnerXP | Canneverbe Limited |
| .AXT | Adobe Systems Incorporated | Settings Files | Adobe Photoshop Extract File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .AZD | Amazon | Settings Files | Amazon Software Download File | Amazon Software Downloader | Amazon |
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