| .I-E | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .I0 | Unknown Developer | Ikke Almindelige Filer | Winter Windows Scheduler File | Unknown Software | Unknown Developer |
| .I3I | IBM | Ikke Almindelige Filer | IBM Voice Type Languages New User File | IBM Voice Type | IBM |
| .I3ML | Unknown Developer | Ikke Almindelige Filer | I3ML Source Page | Unknown Software | Unknown Developer |
| .I66 | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .I8T | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IAC | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IAE | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .IBC | Perion Network Ltd | Ikke Almindelige Filer | IncrediBar Cursor File | IncrediMail | Perion Network Ltd |
| .IBJ | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IBY | Nokia | Ikke Almindelige Filer | Symbian ROM Image Component Include File | Symbian OS | Nokia |
| .IC | elego Software Solutions GmbH | Ikke Almindelige Filer | Modula-3 M3CG Intermediate Language File | Modula-3 | elego Software Solutions GmbH |
| .ICAL | Open Source | Ikke Almindelige Filer | iCalendar File | Microsoft Outlook | Microsoft Corporation |
| .ICALENDAR | Open Source | Ikke Almindelige Filer | iCalendar File | Microsoft Outlook | Microsoft Corporation |
| .ID | IBM | Ikke Almindelige Filer | Lotus Notes User ID File | IBM Lotus Notes | IBM |
| .IDC | Pierre-Emmanuel Gougelet | Ikke Almindelige Filer | Core Software Tech IDC File | XnView MP | XnSoft |
| .IDEAS | DigiSoft Innovations | Ikke Almindelige Filer | An Unknown Apple II File | Golden Orchard | DigiSoft Innovations |
| .IDENTIFIE | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IDLK | Adobe Systems Incorporated | Ikke Almindelige Filer | Adobe InDesign Lock File | Adobe InDesign | Adobe Systems Incorporated |
| .IDQ | Microsoft Corporation | Ikke Almindelige Filer | Microsoft Internet Explorer Data Query File | Internet Explorer | Microsoft Corporation |
| .IDX | Unknown Developer | Ikke Almindelige Filer | Movie Subtitle File | CyberLink PowerDVD | CyberLink |
| .IEU | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IFL | Autodesk, Inc. | Ikke Almindelige Filer | 3ds Max Image File List | Autodesk 3ds Max | Autodesk, Inc. |
| .IFP | Rockstar Games | Ikke Almindelige Filer | GTA Animation File | Grand Theft Auto IV | Rockstar Games |
| .IG | elego Software Solutions GmbH | Ikke Almindelige Filer | Modula-3 Generic Interface Source | Modula-3 | elego Software Solutions GmbH |
| .IGM | Glare Technologies Limited | Ikke Almindelige Filer | Indigo Renderer Material File | Indigo Renderer | Glare Technologies Limited |
| .IGO | Facebook | Ikke Almindelige Filer | Instagram JPEG Image File | Instagram | Facebook |
| .IHR | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .II2 | Gerben Wierda & Tom Hageman | Ikke Almindelige Filer | Remote Installation Format File | i-Installer | Gerben Wierda & Tom Hageman |
| .IIX | Simple DirectMedia Layer (SDL) | Ikke Almindelige Filer | Translation Memory Index File | SDL Trados | SDL Trados |
| .IJM | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IL | Microsoft Corporation | Ikke Almindelige Filer | MSIL Assembler Input File | Microsoft Visual Studio | Microsoft Corporation |
| .ILA | DATEV | Ikke Almindelige Filer | LohnViewer Document | LohnViewer | DATEV |
| .ILI | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IM | Sun Microsystems | Ikke Almindelige Filer | SunOS Grey Scale Bitmap Picture | SunOS | Sun Microsystems |
| .IM?= | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IML | JetBrains | Ikke Almindelige Filer | IntelliJ IDEA Module | Microsoft Notepad | Microsoft Corporation |
| .IMV | Pixela Corporation | Ikke Almindelige Filer | ImageMixerVCD Project File | ImageMixer Mac Edition | Pixela Corporation |
| .IMY | FMJ-Software | Ikke Almindelige Filer | iMelody Ringtone File | Awave Studio | FMJ-Software |
| .IND | Adobe Systems Incorporated | Ikke Almindelige Filer | Adobe InDesign Document | QuarkXPress | Quark Software Inc. |
| .INDIR | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .INE | Foxpertise | Ikke Almindelige Filer | Imagination File | Imagination | Foxpertise |
| .INFO | GNU Project | Ikke Almindelige Filer | Texinfo Document | GNU Emacs | GNU Project |
| .ING | Cosmi Finance LLC | Ikke Almindelige Filer | MasterCook Ingredients File | Mastercook Cookbook | Cosmi Finance LLC |
| .INN | Micro Focus International plc | Ikke Almindelige Filer | Micro Focus COBOL/2 Overlay File | IBM COBOL | IBM |
| .IOF | Microsoft Corporation | Ikke Almindelige Filer | Microsoft FindIt Document | Microsoft FindIt | Microsoft Corporation |
| .IPB | Apple | Ikke Almindelige Filer | iPhone Perl Script | Apple iOS | Apple |
| .IPSW | Apple | Ikke Almindelige Filer | iPod And iPhone Software Update File | Apple iTunes | Apple |
| .IPTC | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .IQ | IBM | Ikke Almindelige Filer | IBM Query Files | Unknown Software | Unknown Developer |
| .IQA | Interscape | Ikke Almindelige Filer | AmeriCalc File | AmeriCalc | TaxCalcUSA |
| .IQB | Interscape | Ikke Almindelige Filer | AmeriCalc File | AmeriCalc | TaxCalcUSA |
| .ISBN | Unknown Developer | Ikke Almindelige Filer | HM3 Tutorial Software File | Unknown Software | Unknown Developer |
| .ISN | Flexera Software | Ikke Almindelige Filer | Installer Source File | InstallShield | Flexera Software |
| .ITC | Apple | Ikke Almindelige Filer | iTunes Cover Flow Data File | Apple iTunes | Apple |
| .ITH | Infraworks Corporation | Ikke Almindelige Filer | InTether File | InTether Receiver | Infraworks Corporation |
| .IUI | Unknown Developer | Ikke Almindelige Filer | Uncommon File Extension | Unknown Software | Unknown Developer |
| .IVA | Unknown Developer | Ikke Almindelige Filer | Surveillance Video File | Unknown Software | Unknown Developer |
| .IVO | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IVP | Oracle Corporation | Ikke Almindelige Filer | Panorama File | Java | Oracle Corporation |
| .IVV | Unknown Developer | Ikke Almindelige Filer | Unknown File Type | Unknown Software | Unknown Developer |
| .IWS | JetBrains | Ikke Almindelige Filer | IntelliJ IDEA Web Page | Internet Explorer | Microsoft Corporation |
| .I_I | Unknown Developer | Ikke Almindelige Filer | Eru/erd File | Unknown Software | Unknown Developer |
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