| .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 |
| .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 |
| .MS-LOCKSCREENCOMPONENT-PRIMARY | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | 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 |
| .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 |
| .M_S | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .N19 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NAI | Unknown Developer | Data Files | WinINSTALL Data | Unknown Software | Unknown Developer |
| .NAK | Unknown Developer | Audio Files | Older Version Audio File | Unknown Software | Unknown Developer |
| .NAMES | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NAPLPS | Unknown Developer | Vector Image Files | North American Presentation Layer Protocol Syntax File | Unknown Software | Unknown Developer |
| .NCP2 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NDJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NDV | Exelis Inc. | Data Files | ENVI N-D Visualizer State Data | Unknown Software | Unknown Developer |
| .NET0143338 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NF | Unknown Developer | Data Files | CVIP NF_help File | Unknown Software | Unknown Developer |
| .NGM | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NII | Unknown Developer | Data Files | NIfTI-1 Data Format | Unknown Software | Unknown Developer |
| .NJP | Unknown Developer | Uncommon Files | NjPipes Compiler File | Unknown Software | Unknown Developer |
| .NNG | Pioneer DJ Corporation | Data Files | Pioneer AVIC Navigation Unit Data | Unknown Software | Unknown Developer |
| .NNX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NOA | Unknown Developer | Video Files | Japanese Movie Format | Unknown Software | Unknown Developer |
| .NOC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NODE1 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NOI | Unknown Developer | 3D Image Files | Tgif-3.0 File | Unknown Software | Unknown Developer |
| .NORMAL | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NOTEPAD | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NPL | Nokia | Data Files | Nokia Playlist File | Unknown Software | Unknown Developer |
| .NSB | NS Basic | Data Files | NS Basic/CE Data | Unknown Software | Unknown Developer |
| .NSM | Unknown Developer | Game Files | NESticle Movie | Unknown Software | Unknown Developer |
| .NT0 | Unknown Developer | Data Files | Adressmn File | Unknown Software | Unknown Developer |
| .NT3 | Unknown Developer | Data Files | Adressmn File | Unknown Software | Unknown Developer |
| .NT_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NUL | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NUWAGE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NV3 | Unknown Developer | Backup Files | Visicontrol MVS-30/35/40 Flash-memory Backup | Unknown Software | Unknown Developer |
| .NWG | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NXS | Unknown Developer | System Files | AdaOS Support Module | Unknown Software | Unknown Developer |
| .NYM | Unknown Developer | Data Files | Shefield Dictionary | Unknown Software | Unknown Developer |
| .OAF | Unknown Developer | Data Files | ETH Oberon Applet | Unknown Software | Unknown Developer |
| .ODIF | ITU-T (International Telecommunication Union, Telecommunication Standardization Sector) | Text Files | Open Document Interchange Format | Unknown Software | Unknown Developer |
| .ODK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .ODTX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .OED | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .OFL_EN | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .OGF2 | Unknown Developer | Data Files | MapViewGPS Map Coordinates File | Unknown Software | Unknown Developer |
| .OLD | Unknown Developer | Backup Files | Backup Archive File | Unknown Software | Unknown Developer |
| .OLV | Unknown Developer | Audio Files | OnLive! Traveler Sound File | Unknown Software | Unknown Developer |
| .OMN | Unknown Developer | Text Files | Omnipotent Collection Data | Unknown Software | Unknown Developer |
| .ON | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ONION | Unknown Developer | Uncommon Files | Dharma Ransomware Affected Data | Unknown Software | Unknown Developer |
| .ONLINE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .ONR | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OPO | Psion PLC | Data Files | Psion OPL Object File | Unknown Software | Unknown Developer |
| .OR | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .ORCA | Unknown Developer | Uncommon Files | Unknown Apple II Data File | Unknown Software | Unknown Developer |
| .ORD | OMAX Corporation | Data Files | OMAX Routed Data | Unknown Software | Unknown Developer |
| .ORI | Unknown Developer | Backup Files | Original File | Unknown Software | Unknown Developer |
| .ORIG | Unknown Developer | Backup Files | Original File | Unknown Software | Unknown Developer |
| .OUB | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OUF | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .OUI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .OUR | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OVB | OnkoS | Backup Files | OnkoS Vocabulary Backup | Unknown Software | Unknown Developer |
| .OWF | OSRAM GmbH | Data Files | OSRAM Waveform File | Unknown Software | Unknown Developer |
| .P11 | HERE | Uncommon Files | HERE Maps For Android Map Data Package | Unknown Software | Unknown Developer |
| .P38 | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .P40 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .P41 | Tandy | Raster Image Files | TRS-80 Image | Unknown Software | Unknown Developer |
| .P56 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .PAN | ProVUE Development | Database Files | Panorama Database File | Unknown Software | Unknown Developer |
| .PARA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .PARAMFILE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .PARSER | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PASCAL | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PATCHER | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PATH | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PAUSE | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PAYROLL | Apple | Data Files | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PBMP | Unknown Developer | Raster Image Files | Renamed UNIX Portable Bitmap Graphic (PBM) File | Unknown Software | Unknown Developer |
| .PC9 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .PC?= | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .PCO | Unknown Developer | Text Files | PC-Outline Outline | Unknown Software | Unknown Developer |
| .PD0 | SonTek | System Files | SonTek Firmware File | Unknown Software | Unknown Developer |
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