| .JUT | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JUX | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JVL | JAVS, Inc. | Uncommon Files | Viewer7.exe | Viewer | JAVS, Inc. |
| .JWC | NTI Corporation | Uncommon Files | JewelCase Maker Saved File | JewelCase Maker | NTI Corporation |
| .JYD | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .K4A | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KAI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KBD | Autodesk, Inc. | Uncommon Files | 3ds Max Keyboard Shortcuts File | Autodesk 3ds Max | Autodesk, Inc. |
| .KBH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KCL | Nintendo | Uncommon Files | Mario Kart Collision File | Mario Kart Wii | Nintendo |
| .KEU | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KFW | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KHS | Open Source | Uncommon Files | Kwalitan History Data File | Kwalitan | Open Source |
| .KIL | Unknown Developer | Uncommon Files | Interview File | Unknown Software | Unknown Developer |
| .KKA | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KKB | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KMAP | Open Source | Uncommon Files | LyX Keymap | Lyx | Open Source |
| .KML | Keyhole Corporation | Uncommon Files | Keyhole Markup Language File | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .KNS | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KOD | Unknown Developer | Uncommon Files | Code File | Unknown Software | Unknown Developer |
| .KOSIMA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KPC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KPK | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KPM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KRY | Herolab | Uncommon Files | Herolab Krypten | Unknown Software | Unknown Developer |
| .KSO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KTS | JetBrains | Uncommon Files | Kotlin Source Code | Android Studio | Google |
| .KWG | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KWR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KXM | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KYR | IBM | Uncommon Files | Lotus Domino Key Ring File | IBM Domino | IBM |
| .KYRA | Emsi Software GmbH | Uncommon Files | Globe Ransomware Affected File | Emsisoft Decrypter for Globe | Emsi Software GmbH |
| .KZY | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .L2L | IBM | Uncommon Files | IBM Voice Type Languages Addword File | IBM Voice Type | IBM |
| .LAP | Don Ho | Uncommon Files | Scheme Source Code | Sublime Text | Sublime HQ Pty Ltd |
| .LAY | Tecplot, Inc | Uncommon Files | Tecplot Layout File | Tecplot 360 | Tecplot, Inc |
| .LBB | Unknown Developer | Uncommon Files | Liberty Bible DataBase File | Unknown Software | Unknown Developer |
| .LCK | Autodesk, Inc. | Uncommon Files | Program Lock File | Adobe Dreamweaver | Adobe Systems Incorporated |
| .LDC | Unknown Developer | Uncommon Files | Steuer 2001 File | Steuer2001 | Unknown Developer |
| .LDH | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LDN | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LEA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LH | Unknown Developer | Uncommon Files | Morph File | Media Code Speed Edit | CDfreaks.com |
| .LIK | Symantec Corporation | Uncommon Files | Trojan.Win32.Agent.Lik Data | Norton Antivirus | Symantec Corporation |
| .LINK | Apple | Uncommon Files | iPod Link File | Apple iOS | Apple |
| .LIU | Trimble Inc. | Uncommon Files | Vico Software License Utilities File | Vico Office Suite | Trimble Inc. |
| .LI_ | Unknown Developer | Uncommon Files | Rukus File | Unknown Software | Unknown Developer |
| .LKH | Unknown Developer | Uncommon Files | Worm.Generic.LKH Virus File | Unknown Software | Unknown Developer |
| .LL3 | LapLink Software | Uncommon Files | LapLink III Document File | LapLink | LapLink Software |
| .LM3 | Boğaziçi University | Uncommon Files | Bosphorus Database Data File | Bosphorus Database | Boğaziçi University |
| .LN | Unknown Developer | Uncommon Files | UNIX Lint Data | Unix | AT&T Labs |
| .LNV | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LOCK | Mozilla | Uncommon Files | Lock File | Mozilla Firefox | Mozilla |
| .LOGONXP | Stardock Corporation | Uncommon Files | LogonStudio Windows XP Logon Screen | LogonStudio | Stardock Corporation |
| .LOJ | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LQC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LRC | Various Developers | Uncommon Files | Lyrics File | Microsoft Windows Media Player | Microsoft Corporation |
| .LRS | Open Source | Uncommon Files | Librie Reader Source File | Calibre | Open Source |
| .LSN | MakeMusic, Inc. | Uncommon Files | Finale Lesson File | Finale | MakeMusic, Inc. |
| .LU | ThoughtWing, LLC | Uncommon Files | ThoughtWing Library Unit File | Unknown Software | Unknown Developer |
| .LUL | Steve Sneed | Uncommon Files | OzWin CompuServe E-mail/Forum Access Upload | OzWin | Steve Sneed |
| .LVH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LVL | Unknown Developer | Uncommon Files | Game Level File | Various video games | Various Developers |
| .LVX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LXA | Microsoft Corporation | Uncommon Files | Microsoft Speech Lexicon File | Microsoft Office | Microsoft Corporation |
| .LYB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LYR | ESRI | Uncommon Files | ArcView Layer File | ESRI ArcGIS for Desktop | ESRI (Environmental Systems Research Institute) |
| .LYZ | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .M5D | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .M9D | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MAB | Mozilla | Uncommon Files | Mozilla Address Book | Apache OpenOffice | Apache Software Foundation |
| .MAD | Electronic Arts | Uncommon Files | Electronic Arts Madcow Movie File | Media Player Classic | Open Source |
| .MAEGZ | Schrödinger, LLC | Uncommon Files | Compressed Molecular Model File | Maestro | Schrödinger, LLC |
| .MAG | Microsoft Corporation | Uncommon Files | Access Diagram Shortcut File | Microsoft Access | Microsoft Corporation |
| .MANIFEST | Microsoft Corporation | Uncommon Files | Windows Application Manifest File | Microsoft Notepad | Microsoft Corporation |
| .MAP | id Software | Uncommon Files | Quake Engine Map File | Valve Hammer Editor | Valve |
| .MAR | Microsoft Corporation | Uncommon Files | Microsoft Access Report | Microsoft Access | Microsoft Corporation |
| .MAS | Image Space Incorporated | Uncommon Files | rFactor Track File | Image Space rFactor | Image Space Incorporated |
| .MAT | MathWorks | Uncommon Files | MATLAB MAT-File | The MathWorks MATLAB | MathWorks |
| .MAU | Unknown Developer | Uncommon Files | Media Attachment Unit | Unknown Software | Unknown Developer |
| .MB | Autodesk, Inc. | Uncommon Files | Maya Binary Project File | Autodesk Maya | Autodesk, Inc. |
| .MB4 | Evan Designs | Uncommon Files | Model Builder File | Model Builder | Evan Designs |
| .MBB | Kodak | Uncommon Files | Kodak EasyShare Data File | Kodak EasyShare | Kodak |
| .MBFAVS | Pierre-Emmanuel Gougelet | Uncommon Files | Stardent AVS X Bitmap Image | XnView MP | XnSoft |
| .MBG | Fookes Software | Uncommon Files | Mailbag Assistant E-mail Archive File | Mailbag Assistant | Fookes Software |
| .MBS | Opera Software | Uncommon Files | Opera Mailbox File | Opera | Opera Software |
| .MC2 | ValuSoft | Uncommon Files | MasterCook 5 Cookbook File | Mastercook Cookbook | Cosmi Finance LLC |
| .MC6 | Smith Micro Software | Uncommon Files | Poser 6 Material File | Poser | Smith Micro Software |
| .MCF | Jehovah’s Witnesses | Uncommon Files | Watchtower Library Font File | Watchtower Library | Jehovah’s Witnesses |
| .MD0 | Alcohol Soft | Uncommon Files | Alcohol Disk Image Segment 1 | Alcohol 120% | Alcohol Soft |
| .MD5 | Ron Rivest | Uncommon Files | MD5 Checksum File | TeraCopy | Code Sector |
| .MDR | Olivier Lapicque | Uncommon Files | ModPlug Compressed Module | ModPlug Player | Open Source |
| .MDW | Microsoft Corporation | Uncommon Files | Access Workgroup File | Microsoft Access | Microsoft Corporation |
| .ME | Unknown Developer | Uncommon Files | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .ME2 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MFC | Unknown Developer | Uncommon Files | Mankind File | Unknown Software | Unknown Developer |
| .MHW | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MIF | Adobe Systems Incorporated | Uncommon Files | FrameMaker Interchange Format File | Adobe FrameMaker | Adobe Systems Incorporated |
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