| acrypt | AutoCrypt Blowfish Encrypted Document |
| acrypx | AutoCrypt AES Encrypted Document |
| acrypz | AutoCrypt AES Encrypted Document |
| acs | Microsoft Agent Character |
| acs2 | AIMP 2 Skin File |
| acs3 | AIMP 3 Skin File |
| acsm | Adobe Content Server Manager Data |
| act | OptionVue Portfolio Manager Account Data |
| act | Adobe Photoshop Color Table |
| act3d | SpinFire Reader ACT3D Data |
| actions | PhotoLine 4 Defaults Data |
| actm | Autodesk AutoCAD Action Macro |
| ac_ | CaseWare Working Papers Compressed Client File |
| ad | After Dark Screensaver |
| adb | Alphacam Diecut VB Macro |
| adb | Intuit Quicken Address Book Data |
| adb | Adlib Express Server Data |
| adc | Active Desktop Calendar Dataset |
| adf | Act! Database File |
| adg | Albatross Design Group 3D Scene |
| adh | Audible Download Helper File |
| adi | AutoCAD Device-Independent Binary Plotter File |
| adif | Amateur Data Interchange Format File |
| adiumlog | Adium Chat Log |
| adix | XML-based ADIF File |
| adj | BenMAP Adjustment Factor File |
| adj | FARSITE Adjustment Factors Data |
| adl | openEHR Archetype Definition Language File |
| adm | Addict Dictionary |
| adm | Windows Administrative Template |
| adm | Alphacam Diecut Param Macro |
| adn | Microsoft Access Blank Project Template |
| adp | Alphacam Diecut Post Data |
| adr | z-Tree Address Data |
| ads | Agricultural Data Interchange Syntax File |