| .CF1 | Würth Online World GmbH | Ficheiros de Dados | WOW! Data | WOW! | Würth Online World GmbH |
| .CF4 | Equi 4 Software | Ficheiros de Dados | Catfish File Manager Support File | CatFish | Equi 4 Software |
| .CFA | Frontline | Ficheiros de Dados | ComProbe Analyzer Capture File | Frontline ComProbe Analysis System | Frontline |
| .CFB | Microsoft Corporation | Ficheiros de Dados | Compound Binary File | Microsoft Visual Studio | Microsoft Corporation |
| .CFH | FieldComm Group | Ficheiros de Dados | Capabilities File For FOUNDATION Fieldbus HSE | Foundation Fieldbus | FieldComm Group |
| .CFL | Corel Corporation | Ficheiros de Dados | Corel Flow Flow Chart File | Corel WordPerfect | Corel |
| .CFU | Symantec Corporation | Ficheiros de Dados | Symantec Client Firewall Policy Data | Client Firewall | Symantec Corporation |
| .CFW | PerkinElmer | Ficheiros de Dados | ChemFinder Form | ChemBioFinder | PerkinElmer |
| .CFX | Flow-Cal, Inc. | Ficheiros de Dados | Flow-Cal Data File | FLOWCAL Enterprise | Flow-Cal, Inc. |
| .CGB | Unknown Developer | Ficheiros de Dados | Gameboy Color Data | Unknown Software | Unknown Developer |
| .CGD | Cricket Technology | Ficheiros de Dados | CricketGraph Data | CricketGraph | Cricket Technology |
| .CGT | Gold Parsing System Team | Ficheiros de Dados | Gold Parsing System Compiled Grammar Table File | GOLD | Gold Parsing System Team |
| .CHE | Procedimientos-Uno, S.L. | Ficheiros de Dados | CHwinEHE Project Files | CHwinEHE | Procedimientos-Uno, S.L. |
| .CHECK | Open Source | Ficheiros de Dados | CASTEP Check Binary Data | CASTEP | Open Source |
| .CHG | Quicken | Ficheiros de Dados | Quicken Online Data File | Intuit Quicken | Quicken |
| .CHI | Microsoft Corporation | Ficheiros de Dados | Microsoft Help File Index | Microsoft Visual FoxPro | Microsoft Corporation |
| .CHK | Sophos | Ficheiros de Dados | InterCheck Checksum Database | Sophos | Sophos |
| .CHL | Leadtek Research | Ficheiros de Dados | WinFast PVR2 Channel List | Leadtek Research WinFast PVR2 | Leadtek Research |
| .CHM | Microsoft Corporation | Ficheiros de Dados | Compiled HTML Help File | Mozilla Firefox | Mozilla |
| .CHN | Qualis Research Associates | Ficheiros de Dados | Ethnograph Data | Qualis Research Associates Ethnograph | Qualis Research Associates |
| .CHP | Corel Corporation | Ficheiros de Dados | Ventura Publisher Chapter Document | Ventura Publisher | Corel |
| .CI3 | Lawtech (Aust.) Pty Ltd | Ficheiros de Dados | Calin Lawyer's Interest Calculator Data File | Calin Lawyer's Interest Calculator | Lawtech (Aust.) Pty Ltd |
| .CIB | DIAL | Ficheiros de Dados | DIALux Luminaire Data | ReluxSuite | Relux |
| .CIE | OxyTech | Ficheiros de Dados | CIE File Format File-Color Information | LITESTAR 4D Photoview | OxyTech |
| .CIF | Corel | Ficheiros de Dados | Easy CD Creator Disk Image | IsoBuster | Smart Projects |
| .CIG | Fifth Generation Systems | Ficheiros de Dados | Fastback Plus Compressed Backup | FastBack Plus | Fifth Generation Systems |
| .CII | Intergraph Corporation | Ficheiros de Dados | Caesar II Data | CAESAR II | Intergraph Corporation |
| .CIK | Corel | Ficheiros de Dados | Corel Graphics 10 Custom Dual Tone File | CorelDRAW Graphics Suite | Corel |
| .CIL | Symantec Corporation | Ficheiros de Dados | Norton CleanSweep Installation Log File | Microsoft Notepad | Microsoft Corporation |
| .CIN | Kodak | Ficheiros de Dados | Kodak Cineon Bitmap File | Adobe SpeedGrade | Adobe Systems Incorporated |
| .CITY | Unknown Developer | Ficheiros de Dados | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .CIX | IBM | Ficheiros de Dados | Rational XDE | Rational XDE | IBM |
| .CKL | DISA (Defense Information Systems Agency) | Ficheiros de Dados | DISA STIG Checklist | STIG (Security Technical Implementation Guides) | DISA (Defense Information Systems Agency) |
| .CKN | Disk Doctor Labs Inc. | Ficheiros de Dados | Disk Doctors License Log | Unknown Software | Unknown Developer |
| .CKT | Altium | Ficheiros de Dados | CircuitMaker File | Altium Designer | Altium |
| .CKY | DragonMinded.com | Ficheiros de Dados | DSOrganize Data | DSOrganize | DragonMinded.com |
| .CL3 | Roxio | Ficheiros de Dados | Easy CD Creator CD Layout | Roxio Creator NXT Pro 5 | Roxio |
| .CL4 | Roxio | Ficheiros de Dados | Easy CD Creator 4 Project File | Roxio Easy CD Creator | Roxio |
| .CLAP | Unknown Developer | Ficheiros de Dados | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .CLB | Microsoft Corporation | Ficheiros de Dados | COM+ Catalog File | Microsoft COM+ | Microsoft Corporation |
| .CLC | Microsoft Corporation | Ficheiros de Dados | Microsoft Schedule+ 7 Habits Mission Statement Data | Microsoft Schedule Plus | Microsoft Corporation |
| .CLE | Moon Valley Software, Inc. | Ficheiros de Dados | Icon Lock-iT Locked File Indicator | Icon Lock-iT | Moon Valley Software, Inc. |
| .CLEANER | Unknown Developer | Ficheiros de Dados | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .CLG | Microsoft Corporation | Ficheiros de Dados | Windows Catalog File | Microsoft Windows | Microsoft Corporation |
| .CLK | Corel | Ficheiros de Dados | Corel R.A.V.E Project File | CorelDRAW Graphics Suite | Corel |
| .CLKP | Crick Software | Ficheiros de Dados | Clicker Palette | Crick software clicker | Crick Software |
| .CLL | Crick Software | Ficheiros de Dados | Crick Software Clicker Data | Crick software clicker | Crick Software |
| .CLM | YMAX | Ficheiros de Dados | MagicJack Call Log File | YMAX magicjack | YMAX |
| .CLO | Pierre-Emmanuel Gougelet | Ficheiros de Dados | Cloe Ray-Tracer Image | XnView MP | XnSoft |
| .CLOG | SpreadMaster | Ficheiros de Dados | SpreadMaster Client Log File | Unknown Software | Unknown Developer |
| .CLONEDVD | Elaborate Bytes | Ficheiros de Dados | CloneDVD Registration Data | elby CloneDVD | Elaborate Bytes |
| .CLOSED | Unknown Developer | Ficheiros de Dados | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .CLP | Reallusion, Inc. | Ficheiros de Dados | CrazyTalk Clip File | CrazyTalk | Reallusion, Inc. |
| .CLT | Clarity Legal | Ficheiros de Dados | Clarity Legal Transcript File | Clarity Legal Viewer | Clarity Legal |
| .CLU | Revolution Software | Ficheiros de Dados | Broken Sword 2 Dialogue Data | Broken Sword 2 | Revolution Software |
| .CLV | Yamaha Corporation | Ficheiros de Dados | Yamaha Cool Voices Data | Yamaha SoundVQ Player | Yamaha Corporation |
| .CLW | Microsoft Corporation | Ficheiros de Dados | Visual C++ ClassWizard File | Microsoft Visual C++ | Microsoft Corporation |
| .CLX | IBM | Ficheiros de Dados | Rational XDE | Rational XDE | IBM |
| .CLY | Corel | Ficheiros de Dados | CorelDRAW Custom Layout | CorelDRAW Graphics Suite | Corel |
| .CM5 | Sci-Ed Software | Ficheiros de Dados | Clone Manager File | SnapGene | GSL Biotech LLC |
| .CM6 | LexisNexis | Ficheiros de Dados | CaseMap Version 6.x Case Analysis File | CaseMap | LexisNexis |
| .CMAP | Institute for Human and Machine Cognition (IHMC) | Ficheiros de Dados | CmapTools Concept Map File | CampTools | Institute for Human and Machine Cognition (IHMC) |
| .CMBL | Vernier Software & Technology | Ficheiros de Dados | Logger Pro Data File | Vernier Logger Pro | Vernier Software & Technology |
| .CMC | Bitz & Pixelz | Ficheiros de Dados | Comic Collector Collection Data File | Comic Collector | Bitz & Pixelz |
| .CMDB | ANSYS, Inc. | Ficheiros de Dados | ANSYS CFX Mesh Database | ANSYS CFX | ANSYS, Inc. |
| .CMDF | CrystalMaker Software | Ficheiros de Dados | CrystalMaker Data File | CrystalMaker | CrystalMaker Software |
| .CML | Unknown Developer | Ficheiros de Dados | Chemical Markup Language File | Avogadro | Open Source |
| .CMN | Microsoft Corporation | Ficheiros de Dados | Systems Management Server (SMS) Update | Microsoft Systems Management Server | Microsoft Corporation |
| .CMP | Microsoft Corporation | Ficheiros de Dados | Windows Connection Manager Profile | Microsoft Windows Server 2016 | Microsoft Corporation |
| .CMR | CMA Coach | Ficheiros de Dados | Coach Results File | CMA Coach | CMA Coach |
| .CMRBK | LexisNexis | Ficheiros de Dados | CaseMap ReportBook Definition File | CaseMap | LexisNexis |
| .CMS | Opera Software | Ficheiros de Dados | Content Management System Web Page File | Safari | Apple |
| .CMY | Corel Corporation | Ficheiros de Dados | CorelDRAW Custom File | CorelDRAW Graphics Suite | Corel |
| .CN | Procedimientos-Uno, S.L. | Ficheiros de Dados | CNwin Project Data | CNwin | Procedimientos-Uno, S.L. |
| .CNA | OpEx Software LLC | Ficheiros de Dados | Catena Project | OpEx Catena | OpEx Software LLC |
| .CNDF | Unknown Developer | Ficheiros de Dados | Technomate CNDF Channel Editor Compressed Channel File | Unknown Software | Unknown Developer |
| .CNJ | Open Source | Ficheiros de Dados | Acharya Conjunct Data | Acharya | Open Source |
| .CNR | David Harris | Ficheiros de Dados | Pegasus Mail Mail Message Stored In Systemwide Folder | Pegasus Mail | David Harris |
| .CNT | Microsoft Corporation | Ficheiros de Dados | Help Contents File | Microsoft HTML Help Workshop | Microsoft Corporation |
| .CNX | IBM | Ficheiros de Dados | IBM Rational XDE Data | Rational XDE | IBM |
| .COBRA | Unknown Developer | Ficheiros de Dados | Unknown Apple II File. (found On Golden Orchard Apple II CD Rom) | Atom | GitHub, Inc. |
| .COF | Microchip Technology Inc. | Ficheiros de Dados | MPLAB COFF File | MPLAB IDE | Microchip Technology Inc. |
| .COK | Eidos Interactive | Ficheiros de Dados | Carmageddon Patch File | Carmageddon TDR 2000 | Torus Games |
| .COL | Open Source | Ficheiros de Dados | Bibexcel File | BibExcel | Open Source |
| .COLLECTION | Adobe Systems Incorporated | Ficheiros de Dados | Adobe Bridge Collection File | Adobe Bridge CC | Adobe Systems Incorporated |
| .COLLOQUYTRANSCRIPT | Javelin.cc | Ficheiros de Dados | Colloquy Chat Transcript File | Colloquy | Open Source |
| .COLORS | Richard Hughey - Kevin Karplus - Anders Krogh | Ficheiros de Dados | SAM Makelogo Color Data | SAM (Sequence Alignment and Modeling) | Richard Hughey - Kevin Karplus - Anders Krogh |
| .CONTACT | Microsoft Corporation | Ficheiros de Dados | Windows Contact File | Microsoft Windows | Microsoft Corporation |
| .CONVERTER | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .CONVERTERX | SiMX | Ficheiros de Dados | SiMX TextConverter Project File | TextConverter | SiMX |
| .COOKIES | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .COREL WORDPERFECT DICTIONARY | Corel | Ficheiros de Dados | Corel WordPerfect Dictionary File | Corel WordPerfect X8 | Corel |
| .CORRECT | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .COUNTER | NetObjects | Ficheiros de Dados | NetObjects Fusion Components Bbscomp Message File | NetObjects Fusion | NetObjects |
| .COVERAGE | Microsoft Corporation | Ficheiros de Dados | Visual Studio Code Coverage Report | Microsoft Visual Studio | Microsoft Corporation |
| .COVERAGEXML | Microsoft Corporation | Ficheiros de Dados | Microsoft Visual Studio Code Coverage File | Microsoft Visual Studio | Microsoft Corporation |
| .CPD | Adobe Systems Incorporated | Ficheiros de Dados | RoboHelp Cache Project Database | Adobe RoboHelp | Adobe Systems Incorporated |
| .CPF | IBM | Ficheiros de Dados | Cognos Project File | IBM Cognos Business Intelligence | IBM |
| .CPI | Sony | Ficheiros de Dados | AVCHD Video Clip Information | Adobe Premiere Pro CC | Adobe Systems Incorporated |
| .CPK | CRI Middleware | Ficheiros de Dados | CRI Middleware Package | CRI Packed File Maker | CRI Middleware |
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