| .KWR | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .KXM | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .KZY | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .L64 | Commodore International | File di settaggi | 64LAN Image File | Unknown Software | Unknown Developer |
| .LARGE | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LASXML | Unknown Developer | File di dati | Laser App Data Exchange | Unknown Software | Unknown Developer |
| .LAUNCHER | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LBB | Unknown Developer | File vari | Liberty Bible DataBase File | Unknown Software | Unknown Developer |
| .LBR | Dag Agren, Circlesoft | File compressi | LU Library Archive | LU file archiver | Unknown Developer |
| .LDC | Unknown Developer | File vari | Steuer 2001 File | Steuer2001 | Unknown Developer |
| .LDH | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LDN | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LDS | GNU Project | File sviluppatore | Binutils LD Linker Script | Unknown Software | Unknown Developer |
| .LEA | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .LED | Sony | File di settaggi | Sony AIBO LED Lights Control Data | Unknown Software | Unknown Developer |
| .LES | David A. Wheeler | File di dati | Lesson File | Unknown Software | Unknown Developer |
| .LEXT | Olympus Corporation | File di dati | Olympus Lext Measurement Export | Unknown Software | Unknown Developer |
| .LG | Unknown Developer | File compressi | Arhangel Compressed Archive | Unknown Software | Unknown Developer |
| .LGF | Microsoft Corporation | File di dati | Microsoft Windows Application Log File | Unknown Software | Unknown Developer |
| .LHD | Unknown Developer | File di testo | Label Home Document File | Label Home | Unknown Developer |
| .LHP | Electric Butterfly, Inc. | File di dati | Lighthouse Organizer Data | Lighthouse Organizer | Unknown Developer |
| .LIB | Various Developers | File di dati | Generic Data Library | Unknown Software | Unknown Developer |
| .LIM | Unknown Developer | File compressi | Limit Compressed Data | Unknown Software | Unknown Developer |
| .LISTING | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LIZ | Unknown Developer | File compressi | Compressed List File | Unknown Software | Unknown Developer |
| .LI_ | Unknown Developer | File vari | Rukus File | Unknown Software | Unknown Developer |
| .LKH | Unknown Developer | File vari | Worm.Generic.LKH Virus File | Unknown Software | Unknown Developer |
| .LKU | STMicroelectronics | File eseguibili | ST Microelectronics Linked Unit Data | Unknown Software | Unknown Developer |
| .LLF | Unknown Developer | File di dati | IITM Local Language Data | IITM | Unknown Developer |
| .LNT | Gimpel Software | File sviluppatore | PC-lint/FlexeLint Configuration File | Laego Visual Manager Live | Unknown Developer |
| .LNV | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LOAD | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOADER | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOAN | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOGO | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LOJ | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LOW | Digital Equipment Corporation | File di sistema | PDP-10 Low Segment Of A Two-segment Program | Unknown Software | Unknown Developer |
| .LP1 | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .LPI | Unknown Developer | File di dati | Laser Printer Information | Unknown Software | Unknown Developer |
| .LPS | DAZ 3D | File 3D | Bryce Leaf Shape File | Unknown Software | Unknown Developer |
| .LQC | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .LQR | Europoe Space Agency (ESA) | File compressi | LBR Library File | LBR library software | Unknown Developer |
| .LSW | Technics | File di settaggi | Technics Keyboard Current Panel File | Unknown Software | Unknown Developer |
| .LSZ | Unknown Developer | File 3D | Learning Score Zip File | Learning Score | Unknown Developer |
| .LU | ThoughtWing, LLC | File vari | ThoughtWing Library Unit File | Unknown Software | Unknown Developer |
| .LVH | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .LVX | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .LYB | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .LYZ | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .LZW | Unknown Developer | File compressi | LZW Compressed Archive File | Unknown Software | Unknown Developer |
| .LZZZZZZZ | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .M | MathWorks | File sviluppatore | MATLAB Source Code File | FMPMD2000 | Unknown Developer |
| .M2 | Unknown Developer | File audio | PC-98 Game Music File | FMPMD2000 | Unknown Developer |
| .M3K | Unknown Developer | File compressi | Interactive Digital Content File | Unknown Software | Unknown Developer |
| .M3V | Unknown Developer | File audio | DJ Mix Pro Playlist | DJ Mix Pro | Unknown Developer |
| .M5D | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .M9D | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .MAC2 | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MACRO | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAILBOX | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAPS | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MARIO | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAU | Unknown Developer | File vari | Media Attachment Unit | Unknown Software | Unknown Developer |
| .MBL | Vernier Software & Technology | File di dati | Logger Pro Data File | Multi-Batch | Unknown Developer |
| .MBZ | Unknown Developer | File con plugin | Pogoshell NES Emulator Plugin | PogoShell | Unknown Developer |
| .MCH | Interscape | File di settaggi | Machinery HDR Effects | Unknown Software | Unknown Developer |
| .MCV | Visicom Media Inc | File di settaggi | ManyCam Effect File | Unknown Software | Unknown Developer |
| .MC_ | Unknown Developer | File compressi | Musicato Compressed File | Unknown Software | Unknown Developer |
| .MD | Various Developers | File sviluppatore | Markdown Documentation File | mdcd10.arc | Unknown Developer |
| .MDL | MathWorks | File di dati | MathWorks Simulink Model | mdl2obj tool | Unknown Developer |
| .MD_ | Unknown Developer | File di settaggi | Visc15 Adressen Setup File | Unknown Software | Unknown Developer |
| .ME | Unknown Developer | File vari | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .ME1 | Unknown Developer | File di dati | 21 Sample Winexam File | Unknown Software | Unknown Developer |
| .ME2 | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEI | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEK | Unknown Developer | File vari | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEMORY | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MERLIN | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MESSAGES | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .METAL | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MFC | Unknown Developer | File vari | Mankind File | Unknown Software | Unknown Developer |
| .MFO | Perl Express Group | File di dati | Perl ModInfo Descriptor File | Unknown Software | Unknown Developer |
| .MGW | Unknown Developer | File di dati | Microgateway Policy Implementation Data | Microgateway | Unknown Developer |
| .MHW | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .MICE | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MISC | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MJM | Unknown Developer | File di dati | JDF Data Exchange Format | Unknown Software | Unknown Developer |
| .MK8 | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .MLI | Unknown Developer | File di dati | Encyclopedia Media List | Unknown Software | Unknown Developer |
| .MLTV | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MMV | Sony | File video | MicroMV Video File | Unknown Software | Unknown Developer |
| .MNW | Unknown Developer | File vari | Menu File | Unknown Software | Unknown Developer |
| .MODEM | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MOJ | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .MOUSE | Apple | File di dati | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MP4S | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .MP6 | Unknown Developer | File di dati | CS225 MP6 Huffman Encoding Text Data | Unknown Software | Unknown Developer |
| .MP_ | Open Source | File audio | Mobile Phone Sound File | P2k Commander | Unknown Developer |
| .MQF | Unknown Developer | File vari | Unknown File Type | Unknown Software | Unknown Developer |
| .MRA | Unknown Developer | File audio | VilagRadio Recorded Radio Program | Unknown Software | Unknown Developer |
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