| aca | Microsoft Agent Character Animation |
| accdu | Microsoft Access Add-in Data |
| accurip | AccuRIP Data |
| acd | MODTRAN ACD Spectral Output File |
| acdb | Android Audio Calibration Database |
| acdb | Aldec Coverage Database |
| ace | Tablet Assembly Data |
| acf | Microsoft Agent Character Data |
| acff | Volocity Library Clipping Data |
| acg | Microsoft Agent Character Preview |
| ach | NACHA ACH File |
| aci | WebCGM ACI File |
| acis | 3D ACIS Modeler Data |
| ack | Adobe Flash Swifty Extension Data |
| acl | Microsoft Office AutoCorrect List |
| aclcddb | Apple Address Book Changelog Database |
| 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 |
| act3d | SpinFire Reader ACT3D Data |
| actions | PhotoLine 4 Defaults Data |
| actm | Autodesk AutoCAD Action Macro |