| .HOX | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .HP5 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HP?= | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HPDATA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HPRT | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .HQB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HR | Atari Corporation | Ficheiros Gráficos | Atari Picture Image File | Unknown Software | Unknown Developer |
| .HRYX | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HUA | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HUF | David A. Huffman | Ficheiros de Dados | Huffman Encoded Data | Unknown Software | Unknown Developer |
| .HVT | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HVUF | Unknown Developer | Ficheiros de Dados | Hydraulic Calculation Format | Unknown Software | Unknown Developer |
| .HWA | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .HWK | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .HX_ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HYB | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .HYD | MARIN | Ficheiros de Base de Dados | MARIN Hydrodynamic Database File | Unknown Software | Unknown Developer |
| .HYL | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .HZ | Unknown Developer | Ficheiros de Texto | Chinese (Hanzi) Text | Unknown Software | Unknown Developer |
| .HZB | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .I-E | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .I0 | Unknown Developer | Uncommon Files | Winter Windows Scheduler File | Unknown Software | Unknown Developer |
| .I3ML | Unknown Developer | Uncommon Files | I3ML Source Page | Unknown Software | Unknown Developer |
| .I66 | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .I8T | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IA | Unknown Developer | Ficheiros Web | Internet Archive | Unknown Software | Unknown Developer |
| .IAC | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IAE | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .IBJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ICN | Microsoft Corporation | Ficheiros de Imagem Raster | Windows Icon File | Unknown Software | Unknown Developer |
| .IDD | Adobe Systems Incorporated | Ficheiros de Layout de Página | Adobe InDesign For Mac Document | Unknown Software | Unknown Developer |
| .IDENTIFIE | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IDK | Anville Instruments | Ficheiros de Dados | Interface Development Kit Data | Unknown Software | Unknown Developer |
| .IDQ | Microsoft Corporation | Ficheiros de Dados | Microsoft Internet Explorer Data Query File | Unknown Software | Unknown Developer |
| .IEE-695 | Hewlett-Packard | Ficheiros de Dados | IEEE 695 Information PostScript File | Unknown Software | Unknown Developer |
| .IEF | Unknown Developer | Ficheiros de Imagem Raster | Bitmap Image Exchange File | Unknown Software | Unknown Developer |
| .IEU | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IFT | Canon Inc. | Ficheiros de Configurações | Canon Settings Data | Unknown Software | Unknown Developer |
| .IGF | Unknown Developer | Ficheiros de Imagem de Vetor | Intercept Geometry Data | Unknown Software | Unknown Developer |
| .IGM | Glare Technologies Limited | Ficheiros de Imagem 3D | Indigo Renderer Material File | Unknown Software | Unknown Developer |
| .IGNORE | Unknown Developer | Ficheiros de Dados | Ignored Data File | Unknown Software | Unknown Developer |
| .IHR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IHT | Unknown Developer | Ficheiros de Desenvolvedor | Intranet Connector Script File | Unknown Software | Unknown Developer |
| .IIM | Ipswitch, Inc. | Ficheiros Executáveis | iMacro Macro File | Unknown Software | Unknown Developer |
| .IJM | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ILG | Flexera Software | Ficheiros de Dados | InstallScript Log File | Unknown Software | Unknown Developer |
| .ILI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .ILL | Unknown Developer | Ficheiros de Texto | Illust Logic Puzzle File | Unknown Software | Unknown Developer |
| .IM?= | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IMW | Unknown Developer | Ficheiros CAD | Imageware Surfacer 3D CAD Surface Geometry File | Unknown Software | Unknown Developer |
| .IM_ | Unknown Developer | Ficheiros de Áudio | Improve Compressed Audio File | Unknown Software | Unknown Developer |
| .INB | LucasArts Entertainment Company, LLC | Ficheiros de Jogo | Rebel Assault Backup | Unknown Software | Unknown Developer |
| .IND | Adobe Systems Incorporated | Ficheiros de Layout de Página | Adobe InDesign Document | Unknown Software | Unknown Developer |
| .INDIR | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .INFINI-D | Unknown Developer | Ficheiros CAD | Infini-D 3.0 Format | Unknown Software | Unknown Developer |
| .INSID | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .INST | Interscape | Ficheiros de Dados | RMCProfile Peak Profile Data | Unknown Software | Unknown Developer |
| .INSTALLER | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .INTEGER | Apple | Ficheiros de Dados | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .IPE | Microsoft Corporation | Ficheiros de Sistema | .ini And Win.ini Backup For Wacom Pen System | Unknown Software | Unknown Developer |
| .IPTC | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .IQ | IBM | Uncommon Files | IBM Query Files | Unknown Software | Unknown Developer |
| .ISBN | Unknown Developer | Uncommon Files | HM3 Tutorial Software File | Unknown Software | Unknown Developer |
| .IT3 | Unknown Developer | Ficheiros de Dados | Internet Timer 3 Log | Unknown Software | Unknown Developer |
| .ITU | Unknown Developer | Ficheiros de Dados | Shefield Dictionary | Unknown Software | Unknown Developer |
| .ITW | Prairie Group | Ficheiros de Base de Dados | InTouch With Database File | Unknown Software | Unknown Developer |
| .IUI | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .IUS | Shanghai InfoTM Microelectronics Co., Ltd. | Ficheiros de Sistema | InfoTMIC Firmware Update File | Unknown Software | Unknown Developer |
| .IVA | Unknown Developer | Ficheiros de Vídeo | Surveillance Video File | Unknown Software | Unknown Developer |
| .IVO | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IVV | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .IXE | Unknown Developer | Ficheiros de Dados | InterEX Control File | Unknown Software | Unknown Developer |
| .I_I | Unknown Developer | Uncommon Files | Eru/erd File | Unknown Software | Unknown Developer |
| .J1 | Unknown Developer | Ficheiros de Dados | EDIT Compiler JEDEC Boundary File | Unknown Software | Unknown Developer |
| .JAVASCRIPT | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .JBX | Unknown Developer | Ficheiros de Dados | Project Scheduler Data | Unknown Software | Unknown Developer |
| .JC5 | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JCAMP | Joint Committee on Atomic and Molecular Physical Data (JCAMP-DX ) | Ficheiros de Dados | JCAMP Standard NMR Data Format | Unknown Software | Unknown Developer |
| .JCB | Jason C. Brooke | Ficheiros de Áudio | CB-JCBrooke Song/Module | Unknown Software | Unknown Developer |
| .JCV | JC Video | Ficheiros de Dados | JC Video Systems Data | Unknown Software | Unknown Developer |
| .JDA | Unknown Developer | Uncommon Files | Winphone Fax Send Journal | Unknown Software | Unknown Developer |
| .JFA | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JGB | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JGG | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .JHTM | Unknown Developer | Uncommon Files | Dynamo Server Page | Unknown Software | Unknown Developer |
| .JMG | Unknown Developer | Ficheiros de Imagem Raster | 16 Bit Adaptive RLE Compressed Bitmap Image | Unknown Software | Unknown Developer |
| .JNJ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .JPK | JPK Instruments | Ficheiros Gráficos | JPK Instruments File Format | Unknown Software | Unknown Developer |
| .JQL | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JRG | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .JSZ | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .JUT | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JUX | Unknown Developer | Uncommon Files | Uncommon File Extension | Unknown Software | Unknown Developer |
| .JVC | JVCKenwood Group | Ficheiros de Vídeo | Digital Camera File | Unknown Software | Unknown Developer |
| .JYD | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .K4A | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KAI | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KBH | Unknown Developer | Uncommon Files | Unknown File Type | Unknown Software | Unknown Developer |
| .KBK | Unknown Developer | Ficheiros de Dados | Medeco XT Codebooks For Keys Data | Unknown Software | Unknown Developer |
| .KBR | Unknown Developer | Ficheiros de Dados | Keirsey-Bates Temperament Sorter Data | Unknown Software | Unknown Developer |
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