| arc | BASIS ASCII Resource Data |
| armi | Carbide ARMI Project |
| arnoldc | ArnoldC Source Code |
| aro | SteelArrow Web Application |
| arq | Remedy ARS Macro |
| arsc | Android Package Resource |
| as | Adobe Flash ActionScript |
| as1 | ActionScript 1.0 Source Code |
| as3 | ActionScript 3.0 Source Code |
| asa | ASP Global Script |
| asax | ASP.NET Application |
| ascp | Adobe Device Central Project |
| ascs | Adobe Device Central Data |
| ascx | ASP.NET User Controls |
| ashx | ASP.NET HTTP Handler |
| asi | Alpha Five Variables Data |
| asic | ASIC Source Code |
| asm | Assembly Source Code |
| asmx | ASP.NET Web Service |
| aso | ActionScript Object |
| asp | Active Server Page |
| aspr | ASProtect Related Data |
| aspx | ASP.NET Page |
| asq | AcuSQL Precompiled File |
| asr | Flash Server-side ActionScript |
| asv | MATLAB AutoSave File |
| asx | Alpha Five Database Index |
| asz | AlphaControls Skin Package |
| atg | Coco/R Attributed Grammar File |
| atsln | Atmel Studio Solution File |
| au3 | AutoIt v3 Script |
| aub | Intel GPU Simulator Input |
| aut | AutoIt Script |
| autosave | GDevelop Autosave Data |
| avm | QAvimator Document |