| aco | ArCon Object Collection |
| aco | CYANA Dihedral Angle Constraint File |
| acq | AcqKnowledge Data |
| acr | ACRobot Script |
| acrodata | Adobe Acrobat Data File |
| 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 |
| ad3e | Adobe Dimensions for Mac Data |
| adb | Intuit Quicken Address Book Data |
| adb | Alphacam Diecut VB Macro |
| 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 |
| adicht | ADInstruments LabChart Chart Data |
| adidat | ADInstruments LabChart Simple Data |
| adif | Amateur Data Interchange Format File |
| adiumlog | Adium Chat Log |
| adix | XML-based ADIF File |
| adj | BenMAP Adjustment Factor File |
| adj | ADS40 Orientation Adjustment Data |