| 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 | PSI-BLAST Checkpoint Profile Data |
| cks | PSI-BLAST Checkpoint Sequence Data |
| ckv | CheckVision Database |
| cky | DSOrganize Data |
| ckz | QuickSFV Checksum Database |
| cl | Carlson Survey Centerline File |
| cl | IconUtils Cursor Library |
| cl2 | Hy-Tek Meet Manager Results Data |
| cl7 | Free Clipboard Viewer Saved Data |
| clam | Adobe InDesign User Dictionary Data |
| classlist | SMART Notebook Class List |
| clb | NoteTab Clipbook Library |
| clb | Corel Mosaic 5.0 Library |
| clb | ICQ Contact List Data |
| clbu | Android Call Log Backup |
| clbx | MP3 Automagic CD Cover Creator Label |
| clc | Corel Mosaic 5.0 Catalogue |
| clct | RootCause Collect Data |
| cld | ClamAV Local Database |
| cldlyr | Terragen Cloud Layer Data |
| clean | Instagram for Android Cache Data |
| clf | ListPro Data |
| clf1 | Common Loudspeaker Format Type 1 File |