| atb | Alphacam Lathe VB Macro |
| atc | AttacheCase Encrypted File |
| atc | Autodesk AutoCAD Tool Catalog |
| atc | AceText Collection File |
| atc_list | aTube Catcher Data |
| atf | ASAM Transfer Format XML Data |
| atf | GenePix Axon Text File |
| atfx | ASAM Transfer Format XML Data |
| ath | Alphacam Lathe Thread |
| 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 |