| 41. | .ac6 | AC/frequence domain output (Spice) |
| 42. | .ac7 | AC/frequence domain output (Spice) |
| 43. | .ac8 | AC/frequence domain output (Spice) |
| 44. | .ac9 | AC/frequence domain output (Spice) |
| 45. | .aca | Animation (Microsoft Agent) ◊ Project (Project Manager Workbench) |
| 46. | .acad | AutoCAD database file |
| 47. | .acb | Compressed file archive (ACB) ◊ Graphics (ACMB) |
| 48. | .acc | Document (Accent Duo 2.0) ◊ Program (DR-DOS - ViewMax) (GEM / resident) |
| 49. | .acd | Character definition file (Microsoft Agent) |
| 50. | .ace | Compressed file archive (ACE, WinACE) |