| acf | Microsoft Agent Character Data |
| acg | Microsoft Agent Character Preview |
| ach | NACHA ACH File |
| aci | WebCGM ACI File |
| acis | 3D ACIS Modeler Data |
| acl | Microsoft Office AutoCorrect List |
| acm | Adobe Photoshop Command Button |
| acml | Aircraft Movement Logger Data |
| 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 |
| actm | Autodesk AutoCAD Action Macro |
| ac_ | CaseWare Working Papers Compressed Client File |
| ad | After Dark Screensaver |
| adb | Adlib Express Server Data |
| adb | Alphacam Diecut VB Macro |
| adb | Intuit Quicken Address Book Data |
| adc | Active Desktop Calendar Dataset |
| adf | Act! Database File |
