| .C V | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .C#= | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .C1A | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .C2 | The POV-Team | Uncommon Files | Pov-Ray For Windows 3.x Scenes Advanced Newltpot Data | Pov-Ray | The POV-Team |
| .C20 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .C2P | Casio Computer Co., Ltd. | Uncommon Files | Casio ClassPad II Picture File | Casio ClassPad ll | Casio Computer Co., Ltd. |
| .C3B | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .C61 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CA | AT&T Labs | Uncommon Files | Initial Telnet Cache | Unix | AT&T Labs |
| .CA0 | Borland | Uncommon Files | Borland Installer Packed & Split File | Borland Installer | Borland |
| .CACHE | Various Developers | Uncommon Files | Cache File | Adobe Photoshop | Adobe Systems Incorporated |
| .CAKEWALKWI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CALENDAR19 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CAMERA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CAO | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CARAMBA | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CASECONVER | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CATANALYSI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CATDRAWIN | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CATMATERIA | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CB3 | Open Source | Uncommon Files | CliBench Mk III SMP File | CliBench mk III smp 0.7.17 | Open Source |
| .CBIN | Unknown Developer | Uncommon Files | CACTVS Chemical Binary File | Unknown Software | Unknown Developer |
| .CBN | j2 Global, Inc. | Uncommon Files | PaperMaster Cabinet File | PaperMaster Pro | j2 Global, Inc. |
| .CBT | Various Developers | Uncommon Files | Comic Book TAR File | Smith Micro StuffIt Deluxe | Smith Micro Software |
| .CBX | IBM | Uncommon Files | Rational XDE Collaboration Data File | Rational XDE | IBM |
| .CCE | Broderbund | Uncommon Files | Calendar Creator 2 Event File | Calendar Creator | Broderbund |
| .CCH | Microsoft Corporation | Uncommon Files | .NET Security Resolution Cache File | Microsoft .NET Framework | Microsoft Corporation |
| .CCL | Open Source | Uncommon Files | CCTools Levelset Data | CCTools | Open Source |
| .CDAT | Microsoft Corporation | Uncommon Files | Internet Security And Acceleration (ISA) Server Cache File | Microsoft Internet Security and Acceleration (ISA) | Microsoft Corporation |
| .CDEV | Apple | Uncommon Files | Macintosh II Control Panel File | macOS | Apple |
| .CDEVSAMPLE | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CDF-MS | Microsoft Corporation | Uncommon Files | ClickOnce Compiled Manifest File | Microsoft ClickOnce | Microsoft Corporation |
| .CDMIA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CDMIC | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CDMID | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CDMIO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CDMIQ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CDN | Avery Dennison | Uncommon Files | Click'N Design 3D CD Label File | Click'N Design 3D | Avery Dennison |
| .CDQ | Open Source | Uncommon Files | Blue Mirror DVD/CD Indexer | Blue Mirror DVD/CD Indexer | Open Source |
| .CDV | Square Box Systems | Uncommon Files | CatDV Catalog | CatDV | Square Box Systems |
| .CE3 | Broderbund | Uncommon Files | Calendar Creator 3.x 4.x Event List File | Calendar Creator | Broderbund |
| .CERBER3 | Malwarebytes | Uncommon Files | Cerber Ransomware Virus File | Kaspersky Anti-Virus | Kaspersky Lab |
| .CES | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .CFK | Microsoft Corporation | Uncommon Files | Microsoft Developer Studio Diagram File | Microsoft Visual Studio | Microsoft Corporation |
| .CFL | Corel Corporation | Uncommon Files | Corel Flow Flow Chart File | Corel WordPerfect | Corel |
| .CFO | Embarcadero Technologies | Uncommon Files | Turbo C Utilities C Form Object File | Turbo C | Borland |
| .CFS | Apache HTTP Project Management Committee | Uncommon Files | Lucene Compound Index File | Apache Lucene | Apache Software Foundation |
| .CHANGELOG | Unknown Developer | Uncommon Files | Project Log Or Record File | Unknown Software | Unknown Developer |
| .CHK | Sophos | Uncommon Files | InterCheck Checksum Database | Sophos | Sophos |
| .CHW | Microsoft Corporation | Uncommon Files | Compiled Help Index File | FAR HTML | Helpware |
| .CHXX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CL | Aha-Soft | Uncommon Files | Cursor Library | Aha-Soft IconUtils | Aha-Soft |
| .CLKK | Crick Software | Uncommon Files | Clicker Keyboard | Crick software clicker | Crick Software |
| .CLR | Adobe Systems Incorporated | Uncommon Files | Animate Color Set | Adobe Animate CC 2017 | Adobe Systems Incorporated |
| .CLS | Leslie Lamport | Uncommon Files | LaTeX Document Class File | MiKTeX | Christian Schenk |
| .CLV | Yamaha Corporation | Uncommon Files | Yamaha Cool Voices Data | Yamaha SoundVQ Player | Yamaha Corporation |
| .CMGZ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CML | Unknown Developer | Uncommon Files | Chemical Markup Language File | Avogadro | Open Source |
| .CMYK | ImageMagick Studio LLC | Uncommon Files | Raw Cyan, Magenta, Yellow And Black Bytes Bitmap Image File | ImageMagick | ImageMagick Studio LLC |
| .CO3 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .COCO | Unknown Developer | Uncommon Files | Unknown Apple II Data File | Unknown Software | Unknown Developer |
| .COF | Microchip Technology Inc. | Uncommon Files | MPLAB COFF File | MPLAB IDE | Microchip Technology Inc. |
| .COM1476662 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .COMICSCART | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .COMMANDLIN | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .COMPLICATI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .COMPRESSIO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CONFIDENTI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CONFIGDBLI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CONTENTSON | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CONTROLPAN | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CONVERSION | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CORELPROJE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CORES | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CORRECTED | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .COS2 | Pinnacle Systems Inc. | Uncommon Files | Pinnacle Studio Cache File | Pinnacle Studio | Corel |
| .CPH_9FA6FA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CPT | Corel | Uncommon Files | Corel Photo-Paint Image | Corel Presentations X8 | Corel |
| .CPU | Open Source | Uncommon Files | Virtual CPU File | CPU Simulation Utility | Open Source |
| .CQ1 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CRA | Unknown Developer | Uncommon Files | Advanced Crack File | Unknown Software | Unknown Developer |
| .CRACKZBYMA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CRC16 | DigiSoft Innovations | Uncommon Files | An Unknown Apple II File | Golden Orchard | DigiSoft Innovations |
| .CRD | Open Source | Uncommon Files | Guitar Tabs | Notepad++ | Don Ho |
| .CRDOWNLOAD | Google | Uncommon Files | Chrome Partially Downloaded File | Google Chrome | Google |
| .CRM | PQ Systems | Uncommon Files | CHARTrunner Multi Chart Document | CHARTrunner Lean | PQ Systems |
| .CRYPTO | Unknown Developer | Uncommon Files | Scarab/CryptoKill Ransomware Virus | Unknown Software | Unknown Developer |
| .CRYPTONOTE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CRYSTALHAM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CSH | Adobe Systems Incorporated | Uncommon Files | Photoshop Custom Shapes File | Adobe Photoshop Elements | Adobe Systems Incorporated |
| .CTF | Interscape | Uncommon Files | WhereIsIt Catalog File | WhereIsIt | Interscape |
| .CTG | Canon Inc. | Uncommon Files | Canon Digital Camera Catalog File | Unknown Software | Unknown Developer |
| .CUID | Unknown Developer | Uncommon Files | Baidu.cuid Data | Unknown Software | Unknown Developer |
| .CUL | CursorArts Company | Uncommon Files | Windows Cursor Library | IconForge | CursorArts Company |
| .CUREXSCHEM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CUSTOM | Commodore International | Uncommon Files | Personal Paint File | Amiga Forever | Cloanto Corporation |
| .CVE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .CVF | CPUID | Uncommon Files | CPU-Z Validation File | CPU-Z | CPUID |
| .CVH | Netmap Software | Uncommon Files | Proprietary Help File | CVTracker | Netmap Software |
| .CVR | Microsoft Corporation | Uncommon Files | Microsoft Crash Report File | Microsoft Excel | Microsoft Corporation |
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