| .JDL | European Grid Infrastructure | Pliki z danymi | gLite Job Description Language File | gLite | European Grid Infrastructure |
| .JDT | Adobe Systems Incorporated | Pliki z danymi | Accelio Capture Classic Filler Data | Accelio Capture Classic Filler | Adobe Systems Incorporated |
| .JDX | Page Scholar Inc | Pliki z danymi | JSpell Dictionary File | JSpell | Page Scholar Inc |
| .JEF | Janome Sewing Machine Co., Ltd. | Pliki z danymi | Janome Embroidery Format | Janome Customizer 11000 | Janome Sewing Machine Co., Ltd. |
| .JFD | Freedom Scientific | Pliki z danymi | JAWS Synopsis Data | JAWS | Freedom Scientific |
| .JGD | Corel | Pliki z danymi | Corel PaintShop Photo Pro Gradients File | PaintShop Pro | Corel |
| .JGF | Freedom Scientific | Pliki z danymi | JAWS Ini File | JAWS | Freedom Scientific |
| .JIO | INRIA | Pliki z danymi | Join-Calculus JCC Compiled Interface | Join-Calculus | INRIA |
| .JLP | Neowin | Pliki z danymi | Jumplist Extender File | Jumplist Extender | Neowin |
| .JMP | SAS Institute Inc. | Pliki z danymi | JMP Data File | SAS | SAS Institute Inc. |
| .JMS | Half-Baked Software | Pliki z danymi | Masher Project | Hot Potatoes | Half-Baked Software |
| .JNL | Actian Corporation | Pliki z danymi | Ingres Journal File | Ingres Database | Actian Corporation |
| .JNT | Microsoft Corporation | Pliki z danymi | Windows Journal File | Microsoft Windows Journal | Microsoft Corporation |
| .JOB | Metamation | Pliki z danymi | MetaCAM Nest Job File | MetaCAM Nesting | Metamation |
| .JOBOPTIONS | Adobe Systems Incorporated | Pliki z danymi | Adobe Joboptions File | Adobe Illustrator CC | Adobe Systems Incorporated |
| .JOC | ConTEXT Project | Pliki z danymi | Context4 DOS Word Processor File | ConTEXT | ConTEXT Project |
| .JOINED | Freebyte | Pliki z danymi | HJ-Split Data File | Freebyte HJSplit for Java | Håkan Gustavsson |
| .JOT | King Stairs | Pliki z danymi | Jot+ Notes Note Data | Jot+ Notes | King Stairs |
| .JP4 | Open Source | Pliki z danymi | Elphel Project Data | Elphel Project | Open Source |
| .JP5 | Electric Quilt | Pliki z danymi | Electric Quilt Data | Electric Quilt | Electric Quilt |
| .JPC | Joint Photographic Experts Group | Pliki z danymi | JPEG 2000 Code Stream File | Adobe Illustrator CC | Adobe Systems Incorporated |
| .JPF | Joint Photographic Experts Group | Pliki z danymi | JPEG 2000 Image | Adobe Illustrator CC | Adobe Systems Incorporated |
| .JPH | Quest Software | Pliki z danymi | JProbe Memory Snapshot File | JProbe | Quest Software |
| .JPL | Sonic Foundry | Pliki z danymi | Siren Jukebox File | Siren Jukebox | Sonic Foundry |
| .JPN | Microsoft Corporation | Pliki z danymi | Windows Japanese Noise-Word List File | Microsoft Notepad | Microsoft Corporation |
| .JPT | Quest Software | Pliki z danymi | Jprobe Threadalyzer Memory Snapshot | JProbe | Quest Software |
| .JPX | Embarcadero Technologies | Pliki z danymi | JBuilder Project | JBuilder | Embarcadero Technologies |
| .JQX | Microsoft Corporation | Pliki z danymi | Microsoft Windows Help-Related File | Microsoft Windows | Microsoft Corporation |
| .JRXML | Jaspersoft | Pliki z danymi | JasperReports Layout File | iReport Designer | Jaspersoft |
| .JSD | Jazer Solutions | Pliki z danymi | Jazer Data File | Jazer | Jazer Solutions |
| .JSF | Adobe Systems Incorporated | Pliki z danymi | Java Script Command File | Adobe Fireworks | Adobe Systems Incorporated |
| .JSK | Cowon Systems | Pliki z danymi | JetAudio File | jetAudio | Cowon Systems |
| .JSM | Mozilla | Pliki z danymi | Firefox JavaScript Module | Mozilla Firefox | Mozilla |
| .JSML | Physiome Project | Pliki z danymi | JSim Model File | JSim | Physiome Project |
| .JSONLD | Microsoft Corporation | Pliki z danymi | Microsoft Visual Studio Data File | Microsoft Visual Studio | Microsoft Corporation |
| .JSPROJ | Microsoft Corporation | Pliki z danymi | Microsoft Visual Studio Data File | Microsoft Visual Studio | Microsoft Corporation |
| .JTB | Wanjun Wang | Pliki z danymi | JTB JavaCC Grammar Data | JTB | Wanjun Wang |
| .JTL | Apache Software Foundation | Pliki z danymi | JMeter Test Results Data File | Apache JMeter | Apache Software Foundation |
| .JW | Microsoft Corporation | Pliki z danymi | Microsoft WordPad Document | Microsoft WordPad | Microsoft Corporation |
| .JWG | Oracle Corporation | Pliki z danymi | Java Data | Java | Oracle Corporation |
| .JWL | Roxio | Pliki z danymi | Roxio Jewel Case File | Roxio Creator NXT Pro 5 | Roxio |
| .JWP | Sonic Solutions | Pliki z danymi | Easy CD Creator Label File | Roxio Easy CD Creator | Roxio |
| .JWR | Rocket Software | Pliki z danymi | LegaSuite GUI J Walk Runtime Data | LegaSuite | Rocket Software |
| .JWRP | Benoit Widemann | Pliki z danymi | JoliWrite File | JoliWrite | Benoit Widemann |
| .JWW | Open Source | Pliki z danymi | Java Web-Workflows Data | Java Web-Workflows | Open Source |
| .JXX | Yamaha Corporation | Pliki z danymi | Yamaha SY77 Audio Data File | Yamaha SY77 | Yamaha Corporation |
| .K02 | SoftVelocity | Pliki z danymi | Clarion Data File | Clarion | SoftVelocity |
| .K2 | Data Access Worldwide | Pliki z danymi | DataFlex Index | DataFlex | Data Access Worldwide |
| .K4 | Data Access Worldwide | Pliki z danymi | DataFlex Index | DataFlex | Data Access Worldwide |
| .K7 | Data Access Worldwide | Pliki z danymi | DataFlex Index | DataFlex | Data Access Worldwide |
| .KAD | Native Instruments | Pliki z danymi | Kore User Data | Kore | Native Instruments |
| .KAL | Kratos Analytical | Pliki z danymi | Kratos Vision Data File | CasaXPS | Casa Software Ltd |
| .KAN | Open Source | Pliki z danymi | Kwalitan Annotation | Kwalitan | Open Source |
| .KAP | Maptech | Pliki z danymi | BSB Chart Image File | Maptech U.S. Boating Charts | Maptech |
| .KAS | Evil Laboratories | Pliki z danymi | Karaoke File | EvilLyrics | Evil Laboratories |
| .KBD | Autodesk, Inc. | Pliki z danymi | 3ds Max Keyboard Shortcuts File | Autodesk 3ds Max | Autodesk, Inc. |
| .KBF | Milenix Software Ltd. | Pliki z danymi | MyInfo Document | MyInfo | Milenix Software Ltd. |
| .KBI | Derek Hendrikz | Pliki z danymi | Keirsey-Bates Temperament Sorter Index | Keirsey-Bates Temperament Sorter | Keirsey Group |
| .KBK | Unknown Developer | Pliki z danymi | Medeco XT Codebooks For Keys Data | Unknown Software | Unknown Developer |
| .KBN | TeamSpeak Systems | Pliki z danymi | TeamSpeak Keybinding File | TeamSpeak | TeamSpeak Systems |
| .KBR | Unknown Developer | Pliki z danymi | Keirsey-Bates Temperament Sorter Data | Unknown Software | Unknown Developer |
| .KBS | PC Dirck | Pliki z danymi | Keyboard Script File | PC Dirck Keyboard Script | PC Dirck |
| .KBX | GNU Project | Pliki z danymi | GNU Privacy Guard Keybox File | GNU Privacy Guard | GNU Project |
| .KCD | Symantec Corporation | Pliki z danymi | Rshiphop.kcd Malware | Avira Antivirus Pro 2016 | Avira Operations GmbH & Co. KG. |
| .KCL | Nintendo | Pliki z danymi | Mario Kart Collision File | Mario Kart Wii | Nintendo |
| .KCM | MM. Sys Bt. | Pliki z danymi | KönyvCalc File | KönyvCalc | MM. Sys Bt. |
| .KCX | Kea Software | Pliki z danymi | Kea Coloring File | Kea Coloring Book | Kea Software |
| .KD3 | Yayoi Co., Ltd. | Pliki z danymi | Yayoi Kaikei Accounting Data | Yayoi Accounting | Yayoi Co., Ltd. |
| .KDBX | Dominik Reichl | Pliki z danymi | KeePass Password Database | KeePass Password Safe | Open Source |
| .KDC | Kaspersky Lab | Pliki z danymi | Kaspersky Virus Database File | Kaspersky Anti-Virus | Kaspersky Lab |
| .KDF | Kinko's | Pliki z danymi | Kinko's File Prep Tool Data | Kinko's File Prep Tool | Kinko's |
| .KDS | Knyzhov Dmitry | Pliki z danymi | KD Player Skin File | Microsoft Notepad | Microsoft Corporation |
| .KDT | National Technical University of Athens | Pliki z danymi | Medbasin Evaporation Data | Medbasin | National Technical University of Athens |
| .KDZ | Kaspersky Lab | Pliki z danymi | Kaspersky Virus Bases File | Kaspersky Anti-Virus | Kaspersky Lab |
| .KEA | Panasonic Corporation | Pliki z danymi | Panasonic KTS File | Panasonic HD Writer AE | Panasonic Corporation |
| .KERNEL | Apple | Pliki z danymi | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .KEY | Unknown Developer | Pliki z danymi | License/ Encrypted Security Key File | Avira Antivirus Pro 2016 | Avira Operations GmbH & Co. KG. |
| .KEYNOTE | Apple | Pliki z danymi | iWork Keynote '06 Presentation | Apple Keynote | Apple |
| .KHF | Open Source | Pliki z danymi | Kwalitan Data File | Kwalitan | Open Source |
| .KID | Inspiration Software Inc. | Pliki z danymi | Kidspiration Document | Kidspiration | Inspiration Software Inc. |
| .KIF | Autodesk, Inc. | Pliki z danymi | AutoCAD Key Index File Format | Autodesk AutoCAD | Autodesk, Inc. |
| .KIK | Apple | Pliki z danymi | Golden Orchard Apple II CD Rom File | Apple OS X | Apple |
| .KIWI | Open Source | Pliki z danymi | KiwiViewer Scene | KiwiViewer | Open Source |
| .KJV | Costas stergiou | Pliki z danymi | TheWord Data | theWord | Costas stergiou |
| .KK2 | Payroll Associates, LLC | Pliki z danymi | ViewChoice | PayChoice | Payroll Associates, LLC |
| .KKT | TTS | Pliki z danymi | TTS-Kone File | TTS-Kone | TTS |
| .KKW | Adobe Systems Incorporated | Pliki z danymi | Adobe RoboHelp Keyword Help Project Index File | Adobe RoboHelp | Adobe Systems Incorporated |
| .KLA | Unknown Developer | Pliki z danymi | KLARFF Map Data | Unknown Software | Unknown Developer |
| .KLB | Software MacKiev | Pliki z danymi | KidPix Sticker, Stamp Album | Kid Pix | Software MacKiev |
| .KLF | Frank Böpple | Pliki z danymi | KaloMa Data | KaloMa | Frank Böpple |
| .KLG | Unknown Developer | Pliki z danymi | KOFIA Log | KOFIA | Unknown Developer |
| .KLI | Unknown Developer | Pliki z danymi | KalKulator Linear Algebra Data | Unknown Software | Unknown Developer |
| .KLIP | Klipfolio Inc. | Pliki z danymi | KlipFolio Klip | KlipFolio | Klipfolio Inc. |
| .KLK | Orgadata AG | Pliki z danymi | Orgadata LogiKal Data | LogiKal | Orgadata AG |
| .KLP | Phoenix Contact | Pliki z danymi | CLIP PROJECT Terminal-Strip Project | Clip Project | Phoenix Contact |
| .KLX | Sinasoft Co. | Pliki z danymi | Kelk 2000 Data | Kelk 2000 | Sinasoft Co. |
| .KM | BMC | Pliki z danymi | BMC Patrol Knowledge Module | BMC | BMC |
| .KM2 | KAISHA-Tec | Pliki z danymi | ActiveModeler Process Map | Avantage | KAISHA-Tec |
| .KMA | Correlate | Pliki z danymi | Correlate Archive File | Correlate | Correlate Client |
| .KMD | Open Source | Pliki z danymi | ABSTAT For Windows Batch File | ABstat | Open Source |
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