| .A-0 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .A021 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .A1J | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .A349 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .A4J | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AA48 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AAS | Adobe Systems Incorporated | Uncommon Files | Adobe Authorware Shocked Packet File | Adobe Authorware | Adobe Systems Incorporated |
| .ABR | Adobe Systems Incorporated | Uncommon Files | Adobe Photoshop Brushes File | Adobe Photoshop | Adobe Systems Incorporated |
| .ACCESS | Sun Microsystems | Uncommon Files | Java Extension File | Java | Oracle Corporation |
| .ACL | Microsoft Corporation | Uncommon Files | AutoCorrect List File | Microsoft Office | Microsoft Corporation |
| .ACS | Microsoft Corporation | Uncommon Files | Agent Character File | Microsoft Agent | Microsoft Corporation |
| .ACTBIN | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ACUTC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AD | Berkeley Systems | Uncommon Files | After Dark Screensaver | After Dark | Berkeley Systems |
| .ADD | Microsoft Corporation | Uncommon Files | Dynamics AX Developer Documentation File | Microsoft Dynamics AX | Microsoft Corporation |
| .ADI | Active@ Data Recovery Software | Uncommon Files | Active@ Disk Image Backup File | Active@ Disk Image | LSoft Technologies Inc. |
| .ADM | Microsoft Corporation | Uncommon Files | Administrative Template File | Microsoft Group Policy Object Editor | Microsoft Corporation |
| .AEG | Unknown Developer | Uncommon Files | Netmino File | Netmino | Unknown Developer |
| .AFAE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AFB | Planit Solutions, Inc. | Uncommon Files | Alphacam Flame VB Macro File | Planit Alphacam | Planit Solutions, Inc. |
| .AFX FILE EXTENSION | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AGD | AgileGraph | Uncommon Files | AgileGraph Data File | AgileGraph | AgileGraph |
| .AHN | NetworkMedia.com, Inc. | Uncommon Files | Our Family Tree Data | Our Family Tree | NetworkMedia.com, Inc. |
| .AHR0CDOVL2 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AIA | MIT Media Lab | Uncommon Files | App Inventor Project | MIT App Inventor | MIT Media Lab |
| .AIF | Apple | Uncommon Files | Audio Interchange File Format | Microsoft Windows Media Player | Microsoft Corporation |
| .AIM | AIMMS B.V. | Uncommon Files | AIMMS ASCII Model File | AIMMS | AIMMS B.V. |
| .AJ0 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .AJO | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .ALT | Microsoft Corporation | Uncommon Files | Dynamics AX Temporary File | Microsoft Dynamics AX | Microsoft Corporation |
| .ANNOT | Adobe Systems Incorporated | Uncommon Files | Adobe Digital Editions Annotations File | Adobe Digital Editions | Adobe Systems Incorporated |
| .AOD | Microsoft Corporation | Uncommon Files | Dynamics AX Object Data File | Microsoft Dynamics AX | Microsoft Corporation |
| .AOL | AOL | Uncommon Files | America Online File | AOL Desktop | AOL |
| .AOR | AOR, Ltd. (Authority on Radio Communications, Ltd.) | Uncommon Files | Radio Scanner Frequency Listing File | Searchlight | AOR, Ltd. (Authority on Radio Communications, Ltd.) |
| .APF | Adobe Systems Incorporated | Uncommon Files | Adobe Profile File | Adobe Acrobat DC | Adobe Systems Incorporated |
| .APG | Unknown Developer | Uncommon Files | Ivana Animated GIF File | Unknown Software | Unknown Developer |
| .ARR | Lemkesoft | Uncommon Files | Amber Graphic File | Lemkesoft GraphicConverter | Lemkesoft |
| .ARU | Symantec Corporation | Uncommon Files | Autorun.aru Malicious Data | Avira Antivirus Pro 2016 | Avira Operations GmbH & Co. KG. |
| .AS4 | askSam Systems | Uncommon Files | AskSam Backup File | askSam Systems SurfSaver | askSam Systems |
| .ASC | Symantec Corporation | Uncommon Files | PGP (Pretty Good Privacy) ASCII Armored File | Mozilla Thunderbird | Mozilla |
| .ASH | Nintendo | Uncommon Files | Nintendo Wii System Menu File | Easy CSM | Open Source |
| .ASS | Ubisoft | Uncommon Files | Splinter Cell Blacklist Game Data File | Splinter Cell Blacklist | Ubisoft |
| .ASXP | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AT | Unknown Developer | Uncommon Files | Rescue Disk File | Unknown Software | Unknown Developer |
| .ATB | AnetSoft | Uncommon Files | AnetHelp Tools File | AnetHelpTool | AnetSoft |
| .ATG | Unknown Developer | Uncommon Files | Netmino File | Netmino | Unknown Developer |
| .ATOS | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ATW | Unknown Developer | Uncommon Files | Attityd Survey Program Data | Unknown Software | Unknown Developer |
| .AUZ | Ableton | Uncommon Files | Ableton Authorization File | Ableton Live | Ableton |
| .AWD | Microsoft Corporation | Uncommon Files | Microsoft Fax Document | PolyView | Polybytes |
| .AWF | Adobe Systems Incorporated | Uncommon Files | Adobe WorkFlow Workgroup Plugin | Adobe Creative Cloud | Adobe Systems Incorporated |
| .AXQ | Richardson Software | Uncommon Files | EditRocket Ada Editor File | Richardson EditRocket | Richardson Software |
| .AYA | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .AYE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .AZ! | Vuze Inc. | Uncommon Files | Vuze Partial Download File | Vuze | Vuze Inc. |
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