| ath | Athena Task File |
| atl | Artlantis 3D Scene |
| atla | Artlantis v5 Archive |
| atlg | SlimBrowser Autologin Data |
| atlo | Artlantis v5 Object |
| atloc | Mac OS X Finder AppleTalk Location Data |
| atls | Artlantis Studio Shader Data |
| atm | RFM Atmospheric Profile Data |
| atm | Alphacam Lathe Param Macro |
| atmc | Adobe Type Manager Data |
| atmn | Automation Anywhere Macro |
| atmo | Adobe Atmosphere World |
| atmsphr | Terragen Atmosphere Landscape Data |
| atn | Adobe Photoshop Actions Data |
| atn | Daqarta Attenuator Calibration Data |
| atomcat | Atom Syndication Category Document |
| atoms | Ribbons Atoms Definition |
| atomsvc | Atom Syndication Service Data |
| atp | Alphacam Lathe Post Data |
| atr | Lightscape Material File |
| atr | PhysioBank Annotation File |
| ats | Advanced ETL Transformation Script |
| att | Alphacam Lathe Tool Data |
| attheme | Android Telegram Theme Data |
| attr | Apple iPhoto Image Attributes |
| atv | Act! Data File |
| atz | Aston Compiled Theme |
| auc | Microsoft Dynamics AX Application Unicode Object Cache |
| audionote | AudioNote Data |
| audx | ApexSQL Audit Architecture File |
| aue | Annotated Urban Environments Data |
| aui | ArcSoft MediaImpression UI Data |
| aul | AviUtl Language File |
| aum | Adobe Update Manager Data File |
| aur | AutoREALM Map |