| cia | Nintendo 3DS CTR Importable Archive |
| cib | Concordance Imagebase File |
| cid | EL-CID Data |
| cie | CIE Color Information Data |
| cif | Crystallographic Information File |
| cif | ColorImpact File |
| cii | Intergraph CAESAR II Data File |
| cik | CorelDRAW Graphics Custom Dual Tone Settings Data |
| cim | Cimplicity HMI Screen |
| cio | SWAT Master Watershed Data |
| cip | Ghost Keylogger Logfile |
| cir | Cadence PSpice Circuit Data |
| cir | Racelogic PerformanceBox Circuit Overlay |
| circ | Logisim Circuit Data |
| cis2 | CIMSteel Integration Standard 2 Document |
| ciw | EndNote Web of Science Export |
| cix | IBM Rational XDE Collaboration Instance File |
| cjb | Autodesk Cleaner XL Job |
| cjstyles | Skin Framework Style Data |
| ck9 | Cook'n Cookbook Backup |
| cka | Comfort Keys Database |
| ckl | DISA STIG Checklist |
| ckn | Disk Doctors CK Support Info File |
| ckp | SQL Server Checkpoint |
| ckv | CheckVision Database |
| cky | DSOrganize Data |
| ckz | QuickSFV Checksum Database |
| cl | Carlson Survey Centerline File |
| cl2 | HY-TEK Meet Results |
| classlist | SMART Notebook Class List |
