| .MRS | MAIET Entertainment | Ficheiros de Jogo | 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 | Ficheiros Compactados | Magic Shadow Archiver Atari Disk Image | Magic Shadow Archiver | Unknown Developer |
| .MSB | Oracle Corporation | Ficheiros de Dados | Oracle Binary Message File | Oracle .msb files unpacker | Unknown Developer |
| .MSEPUB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MSG2 | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MSO | Microsoft Corporation | Ficheiros de Dados | Microsoft Organization Chart Data File | Should Not Be Opened Manually | Unknown Developer |
| .MT9 | Electronics and Telecommunications Research Institute | Ficheiros de Áudio | Music 2.0 Song | Unknown Software | Unknown Developer |
| .MTC | Make The Cut, LLC | Ficheiros de Layout de Página | Make The Cut! Project | Unknown Software | Unknown Developer |
| .MTV | MyMPX.org | Ficheiros de Vídeo | MTV Video Format File | Unknown Software | Unknown Developer |
| .MUK | Unknown Developer | Ficheiros de Áudio | 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 | Ficheiros de Áudio | Audio Data | Unknown Software | Unknown Developer |
| .MVO | Unknown Developer | Ficheiros Gráficos | A Graphic File | Unknown Software | Unknown Developer |
| .MWR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .MWV | Unknown Developer | Ficheiros de Dados | Media Works Data | Unknown Software | Unknown Developer |
| .MW_ | Unknown Developer | Ficheiros de Áudio | Audio Utility Midimach Compressed File | Unknown Software | Unknown Developer |
| .MYW | Unknown Developer | Ficheiros de Imagem de Vetor | MyWorld Education Drawing | Unknown Software | Unknown Developer |
| .MZF | Unknown Developer | Ficheiros Compactados | MediaZip Archive | MediaZip | 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 |
| .N2P | Unknown Developer | Ficheiros de Dados | Net2Phone Data | Net2Phone | Unknown Developer |
| .NAI | Unknown Developer | Ficheiros de Dados | WinINSTALL Data | Unknown Software | Unknown Developer |
| .NAK | Unknown Developer | Ficheiros de Áudio | Older Version Audio File | Unknown Software | Unknown Developer |
| .NAM | Aqua-3D | Ficheiros de Dados | Aqua3D Network File | Aqua3D | Unknown Developer |
| .NAMES | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NAPLPS | Unknown Developer | Ficheiros de Imagem de Vetor | 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. | Ficheiros de Dados | 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 | Ficheiros de Dados | CVIP NF_help File | Unknown Software | Unknown Developer |
| .NFS | Codemasters | Ficheiros de Jogo | F1 2015 Speech Data | Gigawatt NPNFS3 | Unknown Developer |
| .NGM | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .NII | Unknown Developer | Ficheiros de Dados | NIfTI-1 Data Format | Unknown Software | Unknown Developer |
| .NJP | Unknown Developer | Uncommon Files | NjPipes Compiler File | Unknown Software | Unknown Developer |
| .NLP | NirSoft | Ficheiros de Dados | NirLauncher Package | Newsletter Profi 2001 | Unknown Developer |
| .NNG | Pioneer DJ Corporation | Ficheiros de Dados | Pioneer AVIC Navigation Unit Data | Unknown Software | Unknown Developer |
| .NNX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .NOA | Unknown Developer | Ficheiros de Vídeo | 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 | Ficheiros de Imagem 3D | Tgif-3.0 File | Unknown Software | Unknown Developer |
| .NORMAL | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NOTEPAD | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NPL | Nokia | Ficheiros de Dados | Nokia Playlist File | Unknown Software | Unknown Developer |
| .NSB | NS Basic | Ficheiros de Dados | NS Basic/CE Data | Unknown Software | Unknown Developer |
| .NSM | Unknown Developer | Ficheiros de Jogo | NESticle Movie | Unknown Software | Unknown Developer |
| .NT0 | Unknown Developer | Ficheiros de Dados | Adressmn File | Unknown Software | Unknown Developer |
| .NT3 | Unknown Developer | Ficheiros de Dados | 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 | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .NV3 | Unknown Developer | Ficheiros de Cópia de Segurança | 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 | Ficheiros de Sistema | AdaOS Support Module | Unknown Software | Unknown Developer |
| .NYM | Unknown Developer | Ficheiros de Dados | Shefield Dictionary | Unknown Software | Unknown Developer |
| .OAF | Unknown Developer | Ficheiros de Dados | ETH Oberon Applet | Unknown Software | Unknown Developer |
| .ODIF | ITU-T (International Telecommunication Union, Telecommunication Standardization Sector) | Ficheiros de Texto | 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 | Ficheiros de Dados | MapViewGPS Map Coordinates File | Unknown Software | Unknown Developer |
| .OLD | Unknown Developer | Ficheiros de Cópia de Segurança | Backup Archive File | Unknown Software | Unknown Developer |
| .OLV | Unknown Developer | Ficheiros de Áudio | OnLive! Traveler Sound File | Unknown Software | Unknown Developer |
| .OMN | Unknown Developer | Ficheiros de Texto | 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 | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .ONR | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .OOP | Unknown Developer | Ficheiros Compactados | OOP Packer Archive | OOP Packer | Unknown Developer |
| .OPN | Rebel Software | Ficheiros de Jogo | Rebel Strip Poker Opponent Data | Amature Strip Poker | Unknown Developer |
| .OPO | Psion PLC | Ficheiros de Dados | Psion OPL Object File | Unknown Software | Unknown Developer |
| .OR | Apple | Ficheiros de Dados | 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 | Ficheiros de Dados | OMAX Routed Data | Unknown Software | Unknown Developer |
| .ORI | Unknown Developer | Ficheiros de Cópia de Segurança | Original File | Unknown Software | Unknown Developer |
| .ORIG | Unknown Developer | Ficheiros de Cópia de Segurança | 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 | Ficheiros de Cópia de Segurança | OnkoS Vocabulary Backup | Unknown Software | Unknown Developer |
| .OWF | OSRAM GmbH | Ficheiros de Dados | OSRAM Waveform File | Unknown Software | Unknown Developer |
| .P11 | HERE | Uncommon Files | HERE Maps For Android Map Data Package | Unknown Software | Unknown Developer |
| .P38 | Apple | Ficheiros de Dados | 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 | Ficheiros de Imagem Raster | TRS-80 Image | Unknown Software | Unknown Developer |
| .P50 | Unknown Developer | Uncommon Files | The Player Module | The Player | Unknown Developer |
| .P56 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .P61 | Commodore International | Uncommon Files | 6.1a Module File | The Player | Unknown Developer |
| .PAN | ProVUE Development | Ficheiros de Base de Dados | Panorama Database File | Unknown Software | Unknown Developer |
| .PAQ6 | Open Source | Ficheiros Compactados | PAQ6 Data Archive | WinUDA | 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 | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .PASCAL | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
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