| .M | MathWorks | Developer Files | MATLAB Source Code File | FMPMD2000 | Unknown Developer |
| .M2 | Unknown Developer | Audio Files | PC-98 Game Music File | FMPMD2000 | Unknown Developer |
| .M3K | Unknown Developer | Compressed Files | Interactive Digital Content File | Unknown Software | Unknown Developer |
| .M3V | Unknown Developer | Audio Files | 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 | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MACRO | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAILBOX | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAPS | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MARIO | Apple | Data Files | 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 | Data Files | Logger Pro Data File | Multi-Batch | Unknown Developer |
| .MBZ | Unknown Developer | Plugin Files | Pogoshell NES Emulator Plugin | PogoShell | Unknown Developer |
| .MCH | Interscape | Settings Files | Machinery HDR Effects | Unknown Software | Unknown Developer |
| .MCV | Visicom Media Inc | Settings Files | ManyCam Effect File | Unknown Software | Unknown Developer |
| .MC_ | Unknown Developer | Compressed Files | Musicato Compressed File | Unknown Software | Unknown Developer |
| .MD | Various Developers | Developer Files | Markdown Documentation File | mdcd10.arc | Unknown Developer |
| .MDL | MathWorks | Data Files | MathWorks Simulink Model | mdl2obj tool | Unknown Developer |
| .MD_ | Unknown Developer | Settings Files | Visc15 Adressen Setup File | Unknown Software | Unknown Developer |
| .ME | Unknown Developer | Uncommon Files | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .ME1 | Unknown Developer | Data Files | 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 | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MERLIN | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MESSAGES | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .METAL | Apple | Data Files | 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 | Data Files | Perl ModInfo Descriptor File | Unknown Software | Unknown Developer |
| .MGW | Unknown Developer | Data Files | Microgateway Policy Implementation Data | Microgateway | Unknown Developer |
| .MHW | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MICE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MISC | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MJM | Unknown Developer | Data Files | JDF Data Exchange Format | Unknown Software | Unknown Developer |
| .MK8 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MLI | Unknown Developer | Data Files | Encyclopedia Media List | Unknown Software | Unknown Developer |
| .MLTV | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MMV | Sony | Video Files | MicroMV Video File | Unknown Software | Unknown Developer |
| .MNW | Unknown Developer | Uncommon Files | Menu File | Unknown Software | Unknown Developer |
| .MODEM | Apple | Data Files | 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 | Data Files | 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 | Data Files | CS225 MP6 Huffman Encoding Text Data | Unknown Software | Unknown Developer |
| .MP_ | Open Source | Audio Files | Mobile Phone Sound File | P2k Commander | Unknown Developer |
| .MQF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MRA | Unknown Developer | Audio Files | VilagRadio Recorded Radio Program | Unknown Software | Unknown Developer |
| .MRS | MAIET Entertainment | Game Files | GunZ Game Data File | MRS Decompiler | Unknown Developer |
| .MS-LOCKSCREENCOMPONENT-PRIMARY | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MSA | Unknown Developer | Compressed Files | Magic Shadow Archiver Atari Disk Image | Magic Shadow Archiver | Unknown Developer |
| .MSB | Oracle Corporation | Data Files | Oracle Binary Message File | Oracle .msb files unpacker | Unknown Developer |
| .MSEPUB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MSG2 | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MSO | Microsoft Corporation | Data Files | Microsoft Organization Chart Data File | Should Not Be Opened Manually | Unknown Developer |
| .MT9 | Electronics and Telecommunications Research Institute | Audio Files | Music 2.0 Song | Unknown Software | Unknown Developer |
| .MTC | Make The Cut, LLC | Page Layout Files | Make The Cut! Project | Unknown Software | Unknown Developer |
| .MTV | MyMPX.org | Video Files | MTV Video Format File | Unknown Software | Unknown Developer |
| .MUK | Unknown Developer | Audio Files | Multak Mini 8162 Encoded Audio Karaoke | Unknown Software | Unknown Developer |
| .MUV | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MUZIIC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MV3 | Unknown Developer | Audio Files | Audio Data | Unknown Software | Unknown Developer |
| .MVO | Unknown Developer | Graphic Files | A Graphic File | Unknown Software | Unknown Developer |
| .MWR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MWV | Unknown Developer | Data Files | Media Works Data | Unknown Software | Unknown Developer |
| .MW_ | Unknown Developer | Audio Files | Audio Utility Midimach Compressed File | Unknown Software | Unknown Developer |
| .MYW | Unknown Developer | Vector Image Files | MyWorld Education Drawing | Unknown Software | Unknown Developer |
| .MZF | Unknown Developer | Compressed Files | MediaZip Archive | MediaZip | Unknown Developer |
| .M_S | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
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