| .ITX | IBM | Pliki tekstowe | ITX Form Template | FileNet eForms | IBM |
| .IVP | Oracle Corporation | Pliki tekstowe | Panorama File | Java | Oracle Corporation |
| .IWP | Incomedia | Pliki tekstowe | Website X5 Project | Website X5 | Incomedia |
| .IX | Corel | Pliki tekstowe | Corel WordPerfect Office Template File | Corel WordPerfect | Corel |
| .IXT | Image Solutions, Inc. | Pliki tekstowe | ISIPublisher Publication Template | ISIPublisher | Image Solutions, Inc. |
| .JHD | PVR Narasimha Rao | Pliki tekstowe | Jagannatha Hora Document | Jagannatha Hora | PVR Narasimha Rao |
| .JIS | Glenn Rosenthal | Pliki tekstowe | Japanese Industry Standard Text | JWPce | Glenn Rosenthal |
| .JOT | King Stairs | Pliki tekstowe | Jot+ Notes Note Data | Jot+ Notes | King Stairs |
| .JPR | Microsoft Corporation | Pliki tekstowe | WordPad Document | Microsoft WordPad | Microsoft Corporation |
| .JSD | Jazer Solutions | Pliki tekstowe | Jazer Data File | Jazer | Jazer Solutions |
| .JSW | JustSystems | Pliki tekstowe | Ichitaro 4.0 Document | Ichitaro | JustSystems |
| .JTD | JustSystems | Pliki tekstowe | Ichitaro Document | Ichitaro | JustSystems |
| .JWRT | Benoit Widemann | Pliki tekstowe | JoliWrite Data File | JoliWrite | Benoit Widemann |
| .KDF | Kinko's | Pliki tekstowe | Kinko's File Prep Tool Data | Kinko's File Prep Tool | Kinko's |
| .KES | Kurzweil Education | Pliki tekstowe | Kurzweil 3000 Document | Kurzweil 3000 | Kurzweil Education |
| .KNT | SourceForge | Pliki tekstowe | KeyNote Note File | Keynote | SourceForge |
| .KON | Yahoo! | Pliki tekstowe | Yahoo! Widget XML File | Yahoo! Widgets | Yahoo! |
| .KPT | Open Source | Pliki tekstowe | KPresenter Template File | KOffice | Open Source |
| .KTF | Microsoft Corporation | Pliki tekstowe | Microsoft Dynamics AX Kernel Text Data | Microsoft Dynamics AX | Microsoft Corporation |
| .KWD | KDE | Pliki tekstowe | KWord Document | KOffice | Open Source |
| .LAX | AnetSoft | Pliki tekstowe | AnetHelpTool JavaHelp Viewer Data | AnetHelpTool | AnetSoft |
| .LBL | Interscape | Pliki tekstowe | Now Contact Label Template | Now Up-to-Date (Now Contact) | Now Software |
| .LBT | Microsoft Corporation | Pliki tekstowe | FoxPro Label Memo | Microsoft Visual FoxPro | Microsoft Corporation |
| .LDI | Trudi Ersvaer (co-developer) | Pliki tekstowe | LDIF File | Apache Directory Server/Studio | Apache Software Foundation |
| .LEF | Sublime HQ Pty Ltd | Pliki tekstowe | Lenex XML Data | Sublime Text | Sublime HQ Pty Ltd |
| .LET | Genesis 2000, Inc | Pliki tekstowe | Genesis 2000 Data | Genesis 2000 | Genesis 2000, Inc |
| .LGN | Microsoft Corporation | Pliki tekstowe | Microsoft Windows Application Log File | Microsoft Windows | Microsoft Corporation |
| .LHD | Unknown Developer | Pliki tekstowe | Label Home Document File | Label Home | Unknown Developer |
| .LIB | Various Developers | Pliki tekstowe | Generic Data Library | Unknown Software | Unknown Developer |
| .LIS | SourceForge | Pliki tekstowe | SQR (Structured Query Reporting) Output File | Microsoft Notepad | Microsoft Corporation |
| .LNT | Gimpel Software | Pliki tekstowe | PC-lint/FlexeLint Configuration File | Microsoft Notepad | Microsoft Corporation |
| .LOG | Unknown Developer | Pliki tekstowe | Log File | Microsoft Notepad | Microsoft Corporation |
| .LSP | Open Source | Pliki tekstowe | Lisp Program Source Code File | MacroMates TextMate | MacroMates |
| .LST | Unknown Developer | Pliki tekstowe | Data List | LibreOffice | The Document Foundation |
| .LTR | Sublime HQ Pty Ltd | Pliki tekstowe | Letter Document | Microsoft Notepad | Microsoft Corporation |
| .LTX | GSC Game World | Pliki tekstowe | S.T.A.L.K.E.R. Properties File | GSC Game World S.T.A.L.K.E.R.: Call of Pripyat | GSC Game World |
| .LUF | Santanu Ghosh | Pliki tekstowe | Lipikar Uniform Format File | Lipikar | Santanu Ghosh |
| .LWD | EditShare | Pliki tekstowe | Lightworks Data File | Lightworks | EditShare |
| .LWP | IBM | Pliki tekstowe | Lotus Word Pro Document | IBM Lotus Word Pro | IBM |
| .LXFML | The LEGO Group | Pliki tekstowe | LEGO Digital Designer XML File | LEGO Digital Designer | The LEGO Group |
| .LYT | Adminsoft | Pliki tekstowe | Adminsoft Document Layout File | Adminsoft | Adminsoft |
| .LYX | Open Source | Pliki tekstowe | XWE File | XWE | Open Source |
| .M15 | Moving Picture Experts Group | Pliki tekstowe | MPEG Video File | CyberLink PowerDVD | CyberLink |
| .MACWRITE | Apple | Pliki tekstowe | MacWrite Text Document | MacWrite | Apple |
| .MAGNET | Vuze Inc. | Pliki tekstowe | Magnet Link Text File | BitTorrent | BitTorrent |
| .MAN | Unknown Developer | Pliki tekstowe | Unix Manual | LibreOffice | The Document Foundation |
| .MANIFEST | Microsoft Corporation | Pliki tekstowe | Windows Application Manifest File | Microsoft Notepad | Microsoft Corporation |
| .MAP | id Software | Pliki tekstowe | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MDH | Balendu Sharma Dadhich | Pliki tekstowe | Madhyam Document | Madhyam | Balendu Sharma Dadhich |
| .MDK | Microsoft Corporation | Pliki tekstowe | Madoko Document File | Madoko | Microsoft Corporation |
| .MDOWN | John Gruber | Pliki tekstowe | Markdown Markup Language Source Code | Microsoft Notepad | Microsoft Corporation |
| .ME | Unknown Developer | Pliki tekstowe | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .MELL | Mellel | Pliki tekstowe | Mellel Word Processing File | Mellel | Mellel |
| .META4 | JDownloader Team | Pliki tekstowe | Metalink 4.0 Data | Free Download Manager | Free Download Manager.ORG |
| .MIN | Okuma | Pliki tekstowe | Okuma CNC Program File | Microsoft Notepad | Microsoft Corporation |
| .MKD | Malz++Kassner | Pliki tekstowe | CAD 6 Drawing File | CAD6 Studio | Malz++Kassner |
| .MM4 | Microsoft Corporation | Pliki tekstowe | Microsoft Windows Word Processing Format | Microsoft Windows | Microsoft Corporation |
| .MMO | Memory-Map | Pliki tekstowe | Memory Map Overlay File | Memory-Map Navigator | Memory-Map |
| .MNL | Autodesk, Inc. | Pliki tekstowe | AutoCAD Menu LISP File | Autodesk AutoCAD | Autodesk, Inc. |
| .MNT | Microsoft Corporation | Pliki tekstowe | FoxPro Menu Memo | Microsoft Visual FoxPro | Microsoft Corporation |
| .MPL | Sony | Pliki tekstowe | AVCHD Playlist File | CyberLink PowerDVD | CyberLink |
| .MPR | Open Source | Pliki tekstowe | Bibexcel Data | BibExcel | Open Source |
| .MPV | Moving Picture Experts Group | Pliki tekstowe | MPEG Elementary Stream Video File | CyberLink PowerDVD | CyberLink |
| .MRD | Interscape | Pliki tekstowe | MathRevolt Infinity Document | Infinity | Interscape |
| .MSG | Microsoft Corporation | Pliki tekstowe | Saved Email Message | Microsoft Outlook Express | Microsoft Corporation |
| .MSH | Martin Schweiger | Pliki tekstowe | Orbiter 3D Mesh File | Orbiter | Martin Schweiger |
| .MSW | Corel | Pliki tekstowe | Painter Color Mixer Swatches File | Corel Painter | Corel |
| .MTX | Yannick Delwiche | Pliki tekstowe | MadTracker 2 Extension | MadTracker 2 | Yannick Delwiche |
| .MVW | McAfee, Inc. | Pliki tekstowe | Saber LAN Log File | Saber LAN | McAfee, Inc. |
| .MW | Apple | Pliki tekstowe | MacWrite Text Document | MacWrite | Apple |
| .MWD | Mariner Software | Pliki tekstowe | Mariner Write Document | Mariner Write | Mariner Software |
| .MWP | IBM | Pliki tekstowe | Lotus Word Pro SmartMaster File | IBM Lotus Word Pro | IBM |
| .N1E | OM2 Services LLC | Pliki tekstowe | OM2 OMALI NENA Encrypted Document Version 1 | OM2 OMALI Professional | OM2 Services LLC |
| .N3 | Sublime HQ Pty Ltd | Pliki tekstowe | Notation3 Document | Notepad++ | Don Ho |
| .NB | Wolfram Research | Pliki tekstowe | Mathematica Notebook | Wolfram Research Mathematica | Wolfram Research |
| .NCZ | Netcad | Pliki tekstowe | Netcad Drawing | Netcad | Netcad |
| .NE2 | OM2 Services LLC | Pliki tekstowe | OM2 OMALI NENA Document Version 2 | OM2 OMALI Professional | OM2 Services LLC |
| .NFD | Nefsis | Pliki tekstowe | Nefsis Document | Nefsis | Nefsis |
| .NFO | Microsoft Corporation | Pliki tekstowe | System Information File | Microsoft System Information | Microsoft Corporation |
| .NLC | Nero AG | Pliki tekstowe | Nero Linux Compilation File | Nero Linux | Nero AG |
| .NOTE | Failsafe Games | Pliki tekstowe | Notessimo Composition | Notessimo | Starburst |
| .NOTES | Guyswithtowels | Pliki tekstowe | Memento Notes File | Memento | Guyswithtowels |
| .NOW | The Document Foundation | Pliki tekstowe | Readme Text Document File | LibreOffice | The Document Foundation |
| .NPD | Netrosoft Corporation | Pliki tekstowe | Netrosoft Presentation Document | Netrosoft Office Suite | Netrosoft Corporation |
| .NPL | Nokia | Pliki tekstowe | Nokia Playlist File | Nokia Multimedia Player | Nokia |
| .NRF | The Neat Company | Pliki tekstowe | Neat Document Folder File | Neat | The Neat Company |
| .NTG | Mark Milam | Pliki tekstowe | NTGsim Data | NTGsim | Mark Milam |
| .NWM | Sony | Pliki tekstowe | Sony NWM Display Screen File | Adobe Photoshop | Adobe Systems Incorporated |
| .NWP | Perion Network Ltd | Pliki tekstowe | Magentic Wallpaper File | IncrediMail | Perion Network Ltd |
| .OAS | Fujitsu Limited | Pliki tekstowe | Fujitsu OAS Document File | OASYS Viewer | Fujitsu Limited |
| .OCR | URUWORKS | Pliki tekstowe | Subtitle Workshop OCR Script | URUWorks Subtitle Workshop | URUWORKS |
| .ODIF | ITU-T (International Telecommunication Union, Telecommunication Standardization Sector) | Pliki tekstowe | Open Document Interchange Format | Unknown Software | Unknown Developer |
| .ODM | Open Source | Pliki tekstowe | OpenDocument Master Document | Apache OpenOffice | Apache Software Foundation |
| .ODT | Oracle Corporation | Pliki tekstowe | OpenDocument Text Document | Apache OpenOffice | Apache Software Foundation |
| .OLI | Olivetti | Pliki tekstowe | Olivetti Text File | Olitext Plus | Olivetti |
| .OMN | Unknown Developer | Pliki tekstowe | Omnipotent Collection Data | Unknown Software | Unknown Developer |
| .OPT | LexisNexis | Pliki tekstowe | Concordance Database File | LexisNexis Concordance | LexisNexis |
| .ORO | The Three-Mode Company | Pliki tekstowe | 3WayPack T3Rotate Output | 3WayPack | The Three-Mode Company |
| .OSC | OpenStreetMap | Pliki tekstowe | OpenStreetMap Change File | Merkaartor | Bart Vanhauwaert, Chris Browet, Ladislav Láska, and others |
| .OTT | Apache Software Foundation | Pliki tekstowe | OpenDocument Document Template | Apache OpenOffice | Apache Software Foundation |
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