| c1b | Digital Tachograph Data |
| c1s | Capture One Photo Cache Data |
| c1w | Capture One Work Data |
| c2 | POV-Ray Include File |
| c21 | CARE-S InfoWorks Defined Pollutant Detail Data |
| c2db | Intergraph CAESAR II Input Database |
| c2e | NJStar Chinese WP Dictionary Index |
| c2k | Microsoft Project Holidays Data |
| c2l | WinOnCD Data |
| c2m | Mercury C2M Data |
| c2o | Titan Capture Visualiser Data |
| c2r | Windows Media Center Click-to-record File |
| c2s | G7 Click2Stitch Embroidery Design |
| c2v | WellSight Systems HASP SRM License Information Data |
| c3d | Chem3D Template File |
| c3d | Coordinate 3D Data |
| c3d1 | Chem3D Cartesian 1 Data |
| c3d2 | Chem3D Cartesian 2 Data |
| c3e | Cype Data |
| c3l | CommView License Data |
| c3t | Chem3D Template File |
| c3xml | Chem3D XML Format File |
| c3z | OpenQwaq 3D Model |
| c4d | Maxon Cinema 4D Object |
| c4dlib | Maxon Cinema 4D Library |
| c4dupdate | Maxon Cinema 4D Update Data |
| c4p | CrossOver v9 C4P Installer |
| c64 | Commodore 64 ROM Image |
| c64 | PC64 Saved Session |
| c8k | CHIP-8 Keyboard Data |
| ca | DATAIR FlexPlus Data |
| ca | Initial Telnet Cache |
| ca | OpenSSL Certificate Authority Data |
| ca1 | STABCAL Calculation Data |
| ca2 | STABCAL Calculation Data |
