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