| ckm | Creation Kit Macro |
| cko | Kokogut C Output File |
| ckp | Microsoft SQL Server Checkpoint Data |
| cl | Lisp Source Code |
| cl | OpenCL Kernel |
| classpath | Eclipse IDE JAVA Project Classpath Data |
| clb | Code Librarian Database |
| clips | Coda Clips Data |
| clj | Clojure Source Code |
| cljc | Clojure Macros Code |
| cljs | ClojureScript Source Code |
| cljx | Clojure Portable Code |
| clog | MPI Parallel Environment CLOG |
| clojure | Clojure Source Code |
| cloud | Crestron SIMPL Cloud Data |
| cls | Visual Basic Class Module |
| clw | Visual Studio ClassWizard Data |
| clw | Clarion Source Code |
| clz | Crestron SIMPL Library Data |
| cma | OCaml Library |
| cmake | CMake Script |
| cmbx | SharpDevelop Combine File |
| cmc | Crestron SIMPL Module |
| cmd | PTC Creo CL Commands Data |
| cmi | OCaml Compiled Interface |
| cml | ProWorx NXT Comment Data |
| cmn | Visual Studio Windows CE Build Settings |
| cmo | OCaml Compiled Bytecode |
| cmt | OCaml CMT Data |
| cmti | OCaml CMT Data |