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