| .KWR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KXM | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KZY | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .L64 | Commodore International | Ficheiros de Configurações | 64LAN Image File | Unknown Software | Unknown Developer |
| .LARGE | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LASXML | Unknown Developer | Ficheiros de Dados | Laser App Data Exchange | Unknown Software | Unknown Developer |
| .LAUNCHER | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LBB | Unknown Developer | Uncommon Files | Liberty Bible DataBase File | Unknown Software | Unknown Developer |
| .LBR | Dag Agren, Circlesoft | Ficheiros Compactados | LU Library Archive | LU file archiver | Unknown Developer |
| .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 |
| .LDS | GNU Project | Ficheiros de Desenvolvedor | Binutils LD Linker Script | Unknown Software | Unknown Developer |
| .LEA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LED | Sony | Ficheiros de Configurações | Sony AIBO LED Lights Control Data | Unknown Software | Unknown Developer |
| .LES | David A. Wheeler | Ficheiros de Dados | Lesson File | Unknown Software | Unknown Developer |
| .LEXT | Olympus Corporation | Ficheiros de Dados | Olympus Lext Measurement Export | Unknown Software | Unknown Developer |
| .LG | Unknown Developer | Ficheiros Compactados | Arhangel Compressed Archive | Unknown Software | Unknown Developer |
| .LGF | Microsoft Corporation | Ficheiros de Dados | Microsoft Windows Application Log File | Unknown Software | Unknown Developer |
| .LHD | Unknown Developer | Ficheiros de Texto | Label Home Document File | Label Home | Unknown Developer |
| .LHP | Electric Butterfly, Inc. | Ficheiros de Dados | Lighthouse Organizer Data | Lighthouse Organizer | Unknown Developer |
| .LIB | Various Developers | Ficheiros de Dados | Generic Data Library | Unknown Software | Unknown Developer |
| .LIM | Unknown Developer | Ficheiros Compactados | Limit Compressed Data | Unknown Software | Unknown Developer |
| .LISTING | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LIZ | Unknown Developer | Ficheiros Compactados | Compressed List File | Unknown Software | Unknown Developer |
| .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 |
| .LKU | STMicroelectronics | Ficheiros Executáveis | ST Microelectronics Linked Unit Data | Unknown Software | Unknown Developer |
| .LLF | Unknown Developer | Ficheiros de Dados | IITM Local Language Data | IITM | Unknown Developer |
| .LNT | Gimpel Software | Ficheiros de Desenvolvedor | PC-lint/FlexeLint Configuration File | Laego Visual Manager Live | Unknown Developer |
| .LNV | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LOAD | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOADER | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOAN | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOGO | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOJ | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LOW | Digital Equipment Corporation | Ficheiros de Sistema | PDP-10 Low Segment Of A Two-segment Program | Unknown Software | Unknown Developer |
| .LP1 | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LPI | Unknown Developer | Ficheiros de Dados | Laser Printer Information | Unknown Software | Unknown Developer |
| .LPS | DAZ 3D | Ficheiros de Imagem 3D | Bryce Leaf Shape File | Unknown Software | Unknown Developer |
| .LQC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LQR | Europoe Space Agency (ESA) | Ficheiros Compactados | LBR Library File | LBR library software | Unknown Developer |
| .LSW | Technics | Ficheiros de Configurações | Technics Keyboard Current Panel File | Unknown Software | Unknown Developer |
| .LSZ | Unknown Developer | Ficheiros de Imagem 3D | Learning Score Zip File | Learning Score | Unknown Developer |
| .LU | ThoughtWing, LLC | Uncommon Files | ThoughtWing Library Unit File | Unknown Software | Unknown Developer |
| .LVH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LVX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LYB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .LYZ | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LZW | Unknown Developer | Ficheiros Compactados | LZW Compressed Archive File | Unknown Software | Unknown Developer |
| .LZZZZZZZ | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .M | MathWorks | Ficheiros de Desenvolvedor | MATLAB Source Code File | FMPMD2000 | Unknown Developer |
| .M2 | Unknown Developer | Ficheiros de Áudio | PC-98 Game Music File | FMPMD2000 | Unknown Developer |
| .M3K | Unknown Developer | Ficheiros Compactados | Interactive Digital Content File | Unknown Software | Unknown Developer |
| .M3V | Unknown Developer | Ficheiros de Áudio | DJ Mix Pro Playlist | DJ Mix Pro | 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 |
| .MAC2 | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MACRO | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAILBOX | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAPS | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MARIO | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAU | Unknown Developer | Uncommon Files | Media Attachment Unit | Unknown Software | Unknown Developer |
| .MBL | Vernier Software & Technology | Ficheiros de Dados | Logger Pro Data File | Multi-Batch | Unknown Developer |
| .MBZ | Unknown Developer | Ficheiros Plugin | Pogoshell NES Emulator Plugin | PogoShell | Unknown Developer |
| .MCH | Interscape | Ficheiros de Configurações | Machinery HDR Effects | Unknown Software | Unknown Developer |
| .MCV | Visicom Media Inc | Ficheiros de Configurações | ManyCam Effect File | Unknown Software | Unknown Developer |
| .MC_ | Unknown Developer | Ficheiros Compactados | Musicato Compressed File | Unknown Software | Unknown Developer |
| .MD | Various Developers | Ficheiros de Desenvolvedor | Markdown Documentation File | mdcd10.arc | Unknown Developer |
| .MDL | MathWorks | Ficheiros de Dados | MathWorks Simulink Model | mdl2obj tool | Unknown Developer |
| .MD_ | Unknown Developer | Ficheiros de Configurações | Visc15 Adressen Setup File | Unknown Software | Unknown Developer |
| .ME | Unknown Developer | Uncommon Files | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .ME1 | Unknown Developer | Ficheiros de Dados | 21 Sample Winexam 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 |
| .MEMORY | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MERLIN | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MESSAGES | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .METAL | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MFC | Unknown Developer | Uncommon Files | Mankind File | Unknown Software | Unknown Developer |
| .MFO | Perl Express Group | Ficheiros de Dados | Perl ModInfo Descriptor File | Unknown Software | Unknown Developer |
| .MGW | Unknown Developer | Ficheiros de Dados | Microgateway Policy Implementation Data | Microgateway | Unknown Developer |
| .MHW | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MICE | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MISC | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MJM | Unknown Developer | Ficheiros de Dados | JDF Data Exchange Format | Unknown Software | Unknown Developer |
| .MK8 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MLI | Unknown Developer | Ficheiros de Dados | Encyclopedia Media List | Unknown Software | Unknown Developer |
| .MLTV | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MMV | Sony | Ficheiros de Vídeo | MicroMV Video File | Unknown Software | Unknown Developer |
| .MNW | Unknown Developer | Uncommon Files | Menu File | Unknown Software | Unknown Developer |
| .MODEM | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MOJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MOUSE | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MP4S | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MP6 | Unknown Developer | Ficheiros de Dados | CS225 MP6 Huffman Encoding Text Data | Unknown Software | Unknown Developer |
| .MP_ | Open Source | Ficheiros de Áudio | Mobile Phone Sound File | P2k Commander | Unknown Developer |
| .MQF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MRA | Unknown Developer | Ficheiros de Áudio | VilagRadio Recorded Radio Program | Unknown Software | Unknown Developer |
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