| .AAUI | Adobe Systems Incorporated | Plug-in-filer | Acrobat User Interface File | Adobe Acrobat DC | Adobe Systems Incorporated |
| .AAXPLUGIN | Avid Technology | Plug-in-filer | Pro Tools Audio Plugin | Avid Pro Tools | Avid Technology |
| .ABA | Palm | Plug-in-filer | Palm Address Book Archive | Palm Desktop | Palm |
| .ABD | Interscape | Plug-in-filer | Adventure Builder Data | Adventure Builder | Interscape |
| .ABI | Thomas Hall | Plug-in-filer | DNA Chromatogram File | Heracle BioSoft DNA Baser Sequence Assembler | Heracle BioSoft |
| .ACCDA | Microsoft Corporation | Plug-in-filer | Access Add-in File | Microsoft Access | Microsoft Corporation |
| .ACK | Adobe Systems Incorporated | Plug-in-filer | Swifty Data | Adobe Animate CC 2017 | Adobe Systems Incorporated |
| .ADDIN | Microsoft Corporation | Plug-in-filer | Microsoft Visual Studio Add-in | Microsoft Visual Studio | Microsoft Corporation |
| .ADIUMPLUGIN | Adam Iser | Plug-in-filer | Adium Plugin File | Adium | Open Source |
| .ADN | Microsoft Corporation | Plug-in-filer | Access Blank Project Template | Microsoft Access | Microsoft Corporation |
| .AEX | Adobe Systems Incorporated | Plug-in-filer | After Effects Plug-in | Adobe After Effects | Adobe Systems Incorporated |
| .AHQ | Unknown Developer | Plug-in-filer | AudioHQ Plug-in Module | Unknown Software | Unknown Developer |
| .AIP | Adobe Systems Incorporated | Plug-in-filer | Illustrator Plug-in | Adobe Illustrator CC | Adobe Systems Incorporated |
| .ALP | AnyLogic | Plug-in-filer | AnyLogic Project File | AnyLogic | AnyLogic |
| .AMX | Valve | Plug-in-filer | AMX Mod Plugin File | Half-Life | Valve |
| .AMXX | Open Source | Plug-in-filer | AMX Mod X Plugin File | AMX Mod X | Open Source |
| .APD | ACD Systems | Plug-in-filer | ACDSee Photo Document | ACD Systems ACDSee | ACD Systems |
| .APE | Nullsoft | Plug-in-filer | AVS Plugin Effects File | Nullsoft Winamp | Nullsoft |
| .API | Adobe Systems Incorporated | Plug-in-filer | Adobe Photoshop Inks File | Adobe Photoshop CC | Adobe Systems Incorporated |
| .APL | Matthew T. Ashland | Plug-in-filer | Monkey's Audio Track Information File | Microsoft Windows Media Player | Microsoft Corporation |
| .APP | Nokia | Plug-in-filer | Symbian OS Application | Symbian OS | Nokia |
| .ARX | Autodesk, Inc. | Plug-in-filer | AutoCAD Runtime Extension File | Autodesk AutoCAD | Autodesk, Inc. |
| .ASI | Borland | Plug-in-filer | ASsembler Include | Asi Loader | Interscape |
| .ATX | ESRI | Plug-in-filer | ArcGIS Attribute Index File | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .AVX | Avid Technology | Plug-in-filer | Avid Plugin File | Avid Media Composer | Avid Technology |
| .AX | Open Source | Plug-in-filer | DirectShow Filter | Microsoft Windows Media Player | Microsoft Corporation |
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