| atmo | Adobe Atmosphere World |
| atn | Daqarta Attenuator Calibration Data |
| atn | Adobe Photoshop Actions Data |
| atomcat | Atom Syndication Category Document |
| atoms | Ribbons Atoms Definition |
| atomsvc | Atom Syndication Service Data |
| atp | Alphacam Lathe Post Data |
| atr | PhysioBank Annotation File |
| atr | Lightscape Material File |
| ats | Advanced ETL Transformation Script |
| att | Alphacam Lathe Tool Data |
| attr | Apple iPhoto Image Attributes |
| atv | Act! Data File |
| atz | Aston Compiled Theme |
| auc | Microsoft Dynamics AX Application Unicode Object Cache |
| 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 |
| aus | AutoREALM Symbols |
| auz | Ableton Live Authorization |
| ava | Animation Master Model |
| ava | BrainVoyager QX ANCOVA Analysis Data |
| ava3 | AVA CAD/CAM Data |
| avc | Kaspersky Anti-virus Database |
| avd | Avery LabelPro Database |
| avdl | Apache Avro IDL Data |
| avg | AVG Antivirus Virus Database |
