| atoms | Ribbons Atoms Definition |
| atomsvc | Atom Syndication Service Data |
| atp | Alphacam Lathe Post Data |
| atpinf | Automise Project Information |
| atr | Lightscape Material File |
| atr | MicroSim PCBoard Attributes Report |
| atr | PhysioBank Annotation File |
| ats | Advanced ETL Transformation Script |
| ats | Audiotools Plug-in |
| att | Alphacam Lathe Tool Data |
| att | ZipLip Secured E-mail Message |
| att00001 | Microsoft Exchange Generic Attachment Data |
| attheme | Android Telegram Theme Data |
| attr | Apple iPhoto Image Attributes |
| attx | Cadence OrCAD Attributes Data |
| atv | Act! Data File |
| atx | Animation Master Plugin |
| aty | Topicscape Association Type Data |
| atz | Aston Compiled Theme |
| auc | Microsoft Dynamics AX Application Unicode Object Cache |
| aud | GeneMapper Audit Records Data |
| audionote | AudioNote Data |
| audx | ApexSQL Audit Architecture File |
| aue | Annotated Urban Environments Data |
| auf | Audacity Block Data |
| aui | AGE3D UI Editor Color Table |
| aui | ArcSoft MediaImpression UI Data |
| aul | AviUtl Language File |
| aum | Adobe Update Manager Data File |
| aup3-shm | Audacity v3 Temporary Work Data |
